RefreshResumableUploadState uses generic httplib2 UserAgent

Upload and Download classes support passing additional_headers which can include a user-agent, and base_api.py [populates one](https://github.com/google/apitools/blob/b78556b007ec74f8273b0364acf42c3fcf59edaa/apitools/base/py/base_api.py#L615) if it's not [supplied by the client](https://github.com/google/apitools/blob/b78556b007ec74f8273b0364acf42c3fcf59edaa/apitools/base/py/base_api.py#L573).

But RefreshResumableUploadState just creates a [generic http request](https://github.com/google/apitools/blob/b78556b007ec74f8273b0364acf42c3fcf59edaa/apitools/base/py/transfer.py#L777).

This typically makes these refresh requests appear as if they are coming from a different UserAgent than the rest of the Upload.