Do not raise an exception if the header instance is not the correct t…

…ype on app engine. App Engine uses its own httplib, which are (generally) compatible with standard httplib, but with different names. Addresses urllib3#1501.

CONTRIBUTORS.txt

@@ -272,5 +272,8 @@ In chronological order:
 * Justin Bramley <https://github.com/jbramleycl>
   * Add ability to handle multiple Content-Encodings
 
+* Michael Sander (speedplane) <michael.sander@docketalarm.com>
+  * Quiet warning on App Engine.
+
 * [Your name or handle] <[email or website]>
   * [Brief summary of your changes]

src/urllib3/util/response.py

@@ -2,7 +2,7 @@
 from ..packages.six.moves import http_client as httplib
 
 from ..exceptions import HeaderParsingError
-
+from ..contrib import _appengine_environ
 
 def is_fp_closed(obj):
     """
@@ -51,7 +51,8 @@ def assert_header_parsing(headers):
 
     # This will fail silently if we pass in the wrong kind of parameter.
     # To make debugging easier add an explicit check.
-    if not isinstance(headers, httplib.HTTPMessage):
+    if not _appengine_environ.is_appengine() and \
+            not isinstance(headers, httplib.HTTPMessage):
         raise TypeError('expected httplib.Message, got {0}.'.format(
             type(headers)))