lb/TranslationHttpRequestHandler: fix request body leak on cancellation
lb/TranslationHttpRequestHandler: fix request body leak on cancellation

If the client cancels the HTTP request while we're waiting for the
translation server, the request body doesn't get closed.

CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy