Change the content-type to include `vnd.`

[kylef]

Since the specification is registered with IANA, it should therefore use the IANA assigned content type, including the vnd. prefix.

• http://www.iana.org/assignments/media-types/application/vnd.hal+json
• http://www.iana.org/assignments/media-types/application/vnd.hal+xml

/cc @mikekelly

[mikekelly]

This was on purpose. Once the I-D is turned into the RFC and the correct identifier is registered then the IANA registry will no longer be invalid. :)

[mikekelly]

thanks for the PR though, I understand where you're coming from I've discussed this a few times with various people :)