Only encode metadata for user customed headers

Don't encode standard headers keys but just the x-*-meta- ones. That
fixes py3 to not have them casted as bytes and not properly passed to
the requests kwargs.

Other trivial py3 fixes along the way.

Change-Id: I91b95f32fb2aec9b20892a5bb95fd1bc65d002f3
Merge "Only encode metadata for user customed headers"