Update for v2 "API"

[r3pek]

Prosody just released the v2 api. Can this be updated? :)

[weiss]

As far as I can see, the only real change is checking whether the MIME type specified with the XMPP slot request matches the MIME type specified with the HTTP PUT request. I see no real gain in this check, so I'm not sure this warrants the additional complexity of supporting both versions.