Run tests against both HTTP and HTTPS

Some of the tests now use the `httpbin_both` fixture from pytest-httpbin.
Also, made httpbin's CA trusted by default and added `httpbin_secure_untrusted`
fixture  to allow overriding that for particular tests.

tests/test_binary.py

@@ -6,6 +6,7 @@ from fixtures import BIN_FILE_PATH, BIN_FILE_CONTENT, BIN_FILE_PATH_ARG
 
 
 class TestBinaryRequestData:
+
     def test_binary_stdin(self, httpbin):
         with open(BIN_FILE_PATH, 'rb') as stdin:
             env = TestEnvironment(