Make ElasticSearch JSONP response optional.

This adds the http.jsonp.enable option, which is disabled by default. Allowing JSONP responses by default on all API endpoints poses a security risk, and probably shouldn't be used unless it's necessary.

This also squashes a couple bugs in NettyHttpChannel.java
 - 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