Don't treat httplib2.Credentials as oauth credentials (#425)

Author: Jon Wayne Parrott

googleapiclient/_auth.py

@@ -126,7 +126,8 @@ def get_credentials_from_http(http):
         return None
     elif hasattr(http.request, 'credentials'):
         return http.request.credentials
-    elif hasattr(http, 'credentials'):
+    elif (hasattr(http, 'credentials')
+          and not isinstance(http.credentials, httplib2.Credentials)):
         return http.credentials
     else:
         return None