Fixes issue #177

- Provided a ChannelWriter.close(boolean flush) method that provides a close without flush functionality.
- Added a flush in onChannelReadComplete() for ServerRequestResponseConverter.

With this change, if the server's RequestHandler calls response.close(false) then the flush will only be done when the channel read completes.
Fixed failing test.

Any server that processes the request in a different thread, should flush explicitly.
Merge pull request #178 from NiteshKant/master

Fixes issue #177