Fix for issue #126 was wrong.

HttpServerResponse.close() was asserting the transfer encoding to add LastHttpContent.
The transfer encoding was getting added in ServerRequestResponseConverter.
In cases, when close() is NOT invoked from the eventloop of the server, there is a race-condition
between when the transfer encoding is updated (in the handler) and when close() asserts the same as
the handler is invoked in a different thread (server eventloop) than close()

Also added some testcases for this scneario.

Bug fix