Add doHandleWebsocket function

This allows establishing a websocket connection directly in an
HTTPServerRequestDelegate, which is useful if one needs to make some
checks on the request (and possibly send an http response with an error
code) before opening the websocket.

Signed-off-by: Luca Niccoli <l.niccoli@awtech.it>
This does not make much sense with current definition of `Throwable` as application must be completely terminated as soon as it propagates anyway.