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/matchers.py

@@ -1,6 +1,6 @@
 import json
 from six.moves import urllib, xmlrpc_client
-from .util import CaseInsensitiveDict, read_body
+from .util import read_body
 import logging
 
 
@@ -66,9 +66,8 @@ def _identity(x):
 
 
 def _get_transformer(request):
-    headers = CaseInsensitiveDict(request.headers)
     for checker, transformer in _checker_transformer_pairs:
-        if checker(headers): return transformer
+        if checker(request.headers): return transformer
     else:
         return _identity