Uniformize UTF-8 naming (#1115)

* Uniformize UTF-8 naming

Replace `utf8` -> `utf-8` everywhere.
It should have no impact, `utf8` is an alias of `utf-8` [1].

[1] ee03bad25e/Lib/encodings/aliases.py (L534)

* Always specify the encoding

Let's be explicit over implicit. And prevent future warnings from PEP-597 [1].

[1] https://www.python.org/dev/peps/pep-0597/#using-the-default-encoding-is-a-common-mistake

* Update `UTF8` constant (`utf-8` -> `utf_8`)

* Remove default argument from `str.encode()` and `bytes.decode()`

* Clean-up

tests/test_uploads.py

@@ -247,10 +247,10 @@ class TestRequestBodyFromFilePath:
             self, httpbin):
         r = http('--verbose',
                  'POST', httpbin.url + '/post', '@' + FILE_PATH_ARG,
-                 'Content-Type:text/plain; charset=utf8')
+                 'Content-Type:text/plain; charset=UTF-8')
         assert HTTP_OK in r
         assert FILE_CONTENT in r
-        assert 'Content-Type: text/plain; charset=utf8' in r
+        assert 'Content-Type: text/plain; charset=UTF-8' in r
 
     def test_request_body_from_file_by_path_no_field_name_allowed(
             self, httpbin):