Ignore all output after request end

`register_postsend_function` allows user code to be run after the request has
be finished. However, because these functions are just arbitrary user code,
they can produce output themselves. There is no sensible way to handle output
at this stage, and it was causing a failure in the FastCGI transport.

This patch makes transports ignore all output once onSendEnd has be called.
This means that cli behaviour is unchanged.

Fixes #2882