Having content negotiation with more than just JSON content
might follow REST principles.

All entities annotated with e.g. @XmlRootElement can be send as XML data.
This requires client to send Accept header with value "application/json"
to fetch JSON responses.
Default Accept header send by client is something like:
"text/html,application/xhtml+xml,application/xml"
which requests an XML document, not JSON document but JSON is expected.