Only overwrite event host if absent
Set Keep-Alive on all connections that request it

Sets Keep-Alive on HEAD, GET, PUT, POST responses that request it.
Shouldn't really be necessary if the connecting client is using HTTP/1.1
Allow setting of Jetty Connector accept queue size

* Set Jetty Connector accept queue size with config option setAcceptQueueSize
- Default to 0, which is what Jetty default's to, which implicitly uses
  an implementation specific default, which I believe to be 50 on linux
* Note that these changes were branched from master, which still includes
  the Keep-Alive response header in the wrong place as discussed in
  the comments of pull request 2
Add docs for acceptQueueSize, Resolves #3
Forgot to remove Keep-Alive header setting
Merge branch 'master' of https://github.com/moshen/logstash-http-input into jetty-accept-queue