Don't use session.prepare_request

Session.prepare_request didn't exist until requests 2.0 which is newer
than our requirements specifies.

We don't really need this function, we can use the older version for
testing rather than increase our minimum required version.

Change-Id: Ia5a8c187e31b8f883e93d8086481f5e08d764bc7
Closes-Bug: #1361488
Merge "Don't use session.prepare_request"