Require content-type for PUT connections

We should only allow PUT connections if the content-type is explicitly set. This will avoid breakage such as a source sending a ogg/vorbis stream, but not setting a content-type and then being mis-listed as "audio/mpeg" instead of "audio/ogg".

This will need to be made very clear in release notes and documentation, so that people are aware when writing new clients or porting old clients to the PUT protocol to properly set content-type.

We can't enforce this for SOURCE connections as there are simply too many broken clients out there.