Add tests for content-type on JSONP

These should both have a content-type header set to
"application/javascript" per
http://stackoverflow.com/questions/111302/best-content-type-to-serve-jsonp
Fix bug in NettyHttpChannel

1.) We should always set the "application/javascript" content-type on
all JSONP responses, not just disabled ones

2.)  Add some checks (recommended by @spinscale) to make sure
content-type and content-length headers are properly set before sending
the response down the wire.
Pass response object instead of string

This stops a resource leak with the builder never getting closed.