Some media types are extensions of other media types. Such media types often use + in their name to signal this. For example, application/atom+xml extends generic xml and application/hal+json extends generic json.
My question is: what should be done if a client requests the generic media type and the server wants to respond with one of the extended media types? For example, if a request has the header Accept: application/json and the server wants to respond with application/hal+json, should the server...
...deliver a plain-jane JSON with Content-type: application/json, i.e. don't include _links or _embeddeds? That's what the client asked for and that's what it gets. If you wants HAL, ask for it.
...deliver the HAL representation with Content-type: application/json? HAL, after all IS json and that's what the client asked for. The client is happy and can ignore the bits it doesn't understand.
...deliver the HAL representation with Content-type: application/hal+json? Like 2., the client gets what it wants and can ignore the bits it doesn't understand. But there's also a clue that the client can get more out of the representation.
My preference is 3. But is there a spec, best practice, or commonly used approach that can provide guidance which is the best choice?
The server may do any of the three, or it may respond with 406 Not Acceptable if it is unwilling to respond with a default representation.