Add Docker::Util.fix_json.

The "JSON" that is returned by the API isn't parsable. This is because
it's not actually JSON, since there are no delimiters between each hash.
Docker::Util.fix_json attempts to rectify that.
All specs passing with remote api version 1.9.
Add the X-Registry-Config header.

Although this header is only needed for posting to `/build', sending it
with every auth request doesn't hurt and simplifies the code.
Merge pull request #98 from swipely/bump-remote-api-v1.9

Bump remote api v1.9