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.

vcr/stubs/tornado_stubs.py

@@ -15,7 +15,7 @@ def vcr_fetch_impl(cassette, real_fetch_impl):
 
     @functools.wraps(real_fetch_impl)
     def new_fetch_impl(self, request, callback):
-        headers = dict(request.headers)
+        headers = request.headers.copy()
         if request.user_agent:
             headers.setdefault('User-Agent', request.user_agent)