tests: avoid requiring real credentials in unit tests (#459)

Adjust 'Bucket.get_upload_policy' to use credentials from the client,
rather than its connection.

Closes #416.

Author: tseaver

google/cloud/storage/bucket.py

@@ -3181,7 +3181,7 @@ def generate_upload_policy(self, conditions, expiration=None, client=None):
                   to attach the signature.
         """
         client = self._require_client(client)
-        credentials = client._base_connection.credentials
+        credentials = client._credentials
         _signing.ensure_signed_credentials(credentials)
 
         if expiration is None: