Make request.headers always a CaseInsensitiveDict.

Previously request.headers was a normal dict (albeit with the
request.add_header interface) which meant that some code paths would do
case-sensitive matching, for example remove_post_data_parameters which
tests for 'Content-Type'. This change allows all code paths to get the same
case-insensitive treatment.

Additionally request.headers becomes a property to enforce upgrading it to
a CaseInsensitiveDict even if assigned.

tests/integration/test_filter.py

@@ -17,11 +17,7 @@ def _request_with_auth(url, username, password):
 
 
 def _find_header(cassette, header):
-    for request in cassette.requests:
-        for k in request.headers:
-            if header.lower() == k.lower():
-                return True
-    return False
+    return any(header in request.headers for request in cassette.requests)
 
 
 def test_filter_basic_auth(tmpdir):