setting the content type on server error responses

without this they are returned as application/octet-stream which
won't be rendered as html by most modern browsers.
Stream data as fast as possible until the channel indicates it's no longer writable.

This allows us to fill Netty's buffers so that we are less likely to starve the client.
Didn't mean to unignore this.
Merge pull request #417 from tempredirect/bugs/server_error_content_type

setting the content type on server error responses
Add Gareth Davis to contributors.