Prepare for requests 2.0

A small fix for the upcoming native string headers in requests 2.0. We need to re-run prepare_headers as prepare_auth is called after headers inside requests (as it should be).

Tested against current 2.0 branch of requests (py 2.6 - 3.3). As this breaks with requests 1.0 this should be released/merged after 2.0 is on pypi.