RefreshResumableUploadState uses generic httplib2 UserAgent

[thobrla]

Upload and Download classes support passing additional_headers which can include a user-agent, and base_api.py populates one if it's not supplied by the client.

But RefreshResumableUploadState just creates a generic http request.

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

[craigcitro]

👍 should be an easy PR to add in?

[thobrla]

Yes, pretty straightforward.