Set Content-Length for GELF HTTP Input response

Currently, the GELF HTTP input doesn't work without specifying `Connection: close`. Clients will report a timeout when attempting to submit data. This is due to the fact that, when in keep-alive mode, it [must respond with a content-length](http://stackoverflow.com/a/17232225/840849) so the client knows when the data is finished sending. There are some cases where specifying a `Connection: close` header is not possible. Some load balancers strip or change this header when proxying the request.

This PR adds a Content-Length header to the response. Since we never send any data back, the content-length is hardcoded to 0. Apologies for the extraneous commits, they were made before I realized the project is using netty 3.x.

[Tests are confirmed passing.](https://travis-ci.org/meltingice/graylog2-server/builds/23613438)