Adding import of google.auth.credentials to googleapiclient._auth module. (googleapis#328)

Author: wwlian

googleapiclient/_auth.py

@@ -18,6 +18,7 @@
 
 try:
     import google.auth
+    import google.auth.credentials
     import google_auth_httplib2
     HAS_GOOGLE_AUTH = True
 except ImportError:  # pragma: NO COVER