Without this change, the self._response_headers gets a dict_items() type
on PUT. But the rest of the code assumes that it is a list.
Bug manifestation:
File "swift/common/middleware/catch_errors.py", line 120,
in handle_request
self._response_headers.append(('X-Trans-Id', trans_id))
AttributeError: 'dict_items' object has no attribute 'append'
Closes-Bug: #1929083
Change-Id: I5c398b6008716b64c668737e4201ba3b6ab3320b