Response should be flushed on RequestHandler's failure

[NiteshKant]

HttpConnectionHandler does not flush the response on request handling termination to optimize for gathering writes from the request handler.

This makes sense for onComplete() as for graceful completion the RequestHandler must make the decision to flush or not. However, for error, specifically for unexpected errors this default (of not flushing the response) does not make sense as the error could have happened even before the handler had chance to populate/handle the response. In a case when the request handling is done in a different thread* and failed without flushing the response, the response would not be flushed unless some other code invokes flush() on the same connection (in response to a different request).

* The reason why this works for request handling done in the server eventloop is that we do an explicit flush at the channel read complete.

[NiteshKant]

PR #227 fixes this issue by flushing the response in case of an error from RequestHandler