Correct Hal mime type

BREAKING CHANGE: "hal" media type now specifies a json response.

To migrate your code change existing usage of "hal" media type to
"halvnd".

- Allow for both json and xml Hal content types
- Remove use of "vnd" in the subtype to make it conform to
  https://tools.ietf.org/html/draft-kelly-json-hal-06#section-3