Do not decode headers in v1/images.py

v1.images._image_meta_to_headers currently encodes headers as a way to
ensure they're an instance of basestring. This is not necessary since
headers will be encoded later during the request. Also, all data within
the client should be already decoded.

Fixes bug: #1187013

Change-Id: I80525adbc6e9e576cfad5b576090ef9ee574c1cf
Merge "Do not decode headers in v1/images.py"