diff --git a/httpie/client.py b/httpie/client.py
index b79cb1be..bb5410de 100644
--- a/httpie/client.py
+++ b/httpie/client.py
@@ -1,19 +1,20 @@
+import argparse
+import http.client
 import json
 import sys
-
-import http.client
-import requests
+import zlib
 from contextlib import contextmanager
-from requests.adapters import HTTPAdapter
-from requests.structures import CaseInsensitiveDict
+from pathlib import Path
 
-from httpie import sessions
-from httpie import __version__
+import requests
+from requests.adapters import HTTPAdapter
+
+from httpie import __version__, sessions
 from httpie.cli.constants import SSL_VERSION_ARG_MAPPING
+from httpie.cli.dicts import RequestHeadersDict
 from httpie.plugins import plugin_manager
 from httpie.utils import repr_dict_nice
 
-import zlib
 
 try:
     # https://urllib3.readthedocs.io/en/latest/security.html
@@ -31,7 +32,7 @@ except (ImportError, AttributeError):
 FORM_CONTENT_TYPE = 'application/x-www-form-urlencoded; charset=utf-8'
 JSON_CONTENT_TYPE = 'application/json'
 JSON_ACCEPT = f'{JSON_CONTENT_TYPE}, */*'
-DEFAULT_UA = 'HTTPie/%s' % __version__
+DEFAULT_UA = f'HTTPie/{__version__}'
 
 
 # noinspection PyProtectedMember
@@ -48,46 +49,40 @@ def max_headers(limit):
 
 class HTTPieHTTPAdapter(HTTPAdapter):
 
-    def __init__(self, ssl_version=None, **kwargs):
+    def __init__(self, ssl_version=None, compress=0, **kwargs):
         self._ssl_version = ssl_version
+        self._compress = compress
         super().__init__(**kwargs)
 
     def init_poolmanager(self, *args, **kwargs):
         kwargs['ssl_version'] = self._ssl_version
         super().init_poolmanager(*args, **kwargs)
 
-
-class ContentCompressionHttpAdapter(HTTPAdapter):
-
-    def __init__(self, compress, **kwargs):
-        self.compress = compress
-        super().__init__(**kwargs)
-
-    def send(self, request, **kwargs):
-        if request.body and self.compress > 0:
-            deflater = zlib.compressobj()
-            if isinstance(request.body, bytes):
-                deflated_data = deflater.compress(request.body)
-            else:
-                deflated_data = deflater.compress(request.body.encode())
-            deflated_data += deflater.flush()
-            if len(deflated_data) < len(request.body) or self.compress > 1:
-                request.body = deflated_data
-                request.headers['Content-Encoding'] = 'deflate'
-                request.headers['Content-Length'] = str(len(deflated_data))
+    def send(self, request: requests.PreparedRequest, **kwargs):
+        if self._compress and request.body:
+            self._compress_body(request, self._compress)
         return super().send(request, **kwargs)
 
+    @staticmethod
+    def _compress_body(request: requests.PreparedRequest, compress: int):
+        deflater = zlib.compressobj()
+        if isinstance(request.body, bytes):
+            deflated_data = deflater.compress(request.body)
+        else:
+            deflated_data = deflater.compress(request.body.encode())
+        deflated_data += deflater.flush()
+        if len(deflated_data) < len(request.body) or compress > 1:
+            request.body = deflated_data
+            request.headers['Content-Encoding'] = 'deflate'
+            request.headers['Content-Length'] = str(len(deflated_data))
 
-def get_requests_session(ssl_version, compress):
+
+def get_requests_session(ssl_version: str, compress: int) -> requests.Session:
     requests_session = requests.Session()
-    requests_session.mount(
-        'https://',
-        HTTPieHTTPAdapter(ssl_version=ssl_version)
-    )
-    if compress:
-        adapter = ContentCompressionHttpAdapter(compress)
-        for prefix in ['http://', 'https://']:
-            requests_session.mount(prefix, adapter)
+    adapter = HTTPieHTTPAdapter(ssl_version=ssl_version, compress=compress)
+    for prefix in ['http://', 'https://']:
+        requests_session.mount(prefix, adapter)
+
     for cls in plugin_manager.get_transport_plugins():
         transport_plugin = cls()
         requests_session.mount(prefix=transport_plugin.prefix,
@@ -95,7 +90,10 @@ def get_requests_session(ssl_version, compress):
     return requests_session
 
 
-def get_response(args, config_dir):
+def get_response(
+    args: argparse.Namespace,
+    config_dir: Path
+) -> requests.Response:
     """Send the request and return a `request.Response`."""
 
     ssl_version = None
@@ -123,32 +121,29 @@ def get_response(args, config_dir):
     return response
 
 
-def dump_request(kwargs):
+def dump_request(kwargs: dict):
     sys.stderr.write('\n>>> requests.request(**%s)\n\n'
                      % repr_dict_nice(kwargs))
 
 
-def finalize_headers(headers):
-    final_headers = {}
+def finalize_headers(headers: RequestHeadersDict) -> RequestHeadersDict:
+    final_headers = RequestHeadersDict()
     for name, value in headers.items():
         if value is not None:
-
             # >leading or trailing LWS MAY be removed without
             # >changing the semantics of the field value"
             # -https://www.w3.org/Protocols/rfc2616/rfc2616-sec4.html
             # Also, requests raises `InvalidHeader` for leading spaces.
             value = value.strip()
-
             if isinstance(value, str):
                 # See: https://github.com/jakubroztocil/httpie/issues/212
                 value = value.encode('utf8')
-
         final_headers[name] = value
     return final_headers
 
 
-def get_default_headers(args):
-    default_headers = CaseInsensitiveDict({
+def get_default_headers(args: argparse.Namespace) -> RequestHeadersDict:
+    default_headers = RequestHeadersDict({
         'User-Agent': DEFAULT_UA
     })
 
@@ -165,7 +160,7 @@ def get_default_headers(args):
     return default_headers
 
 
-def get_requests_kwargs(args, base_headers=None):
+def get_requests_kwargs(args: argparse.Namespace, base_headers=None) -> dict:
     """
     Translate our `args` into `requests.request` keyword arguments.