Add get_all to compatibility function

This is used in python 3 rather that the getheaders function that we
already stub out.

Change-Id: Ie147214a49fc744dd2ff1fc958c60547874d05d9
Closes-Bug: #1361527

requests_mock/compat.py

@@ -31,6 +31,10 @@ if _requests_version < (2, 3):
         def getheaders(self, name):
             return None
 
+        def get_all(self, name, failobj=None):
+            # python 3 only, overrides email.message.Message.get_all
+            return failobj
+
     class _FakeHTTPResponse(object):
 
         def __init__(self):