REST API: Allow to configure JSONP/callback support

Added the http.jsonp.enable option to configure disabling of JSONP responses, as those
might pose a security risk, and can be disabled if unused.

This also fixes bugs in NettyHttpChannel
* JSONP responses were never setting application/javascript as the content-type
* The content-type and content-length headers were being overwritten even if they were set before

Closes #6164