BUG/MINOR: mux-h2: send a CANCEL instead of ES on truncated writes

BUG/MINOR: mux-h2: send a CANCEL instead of ES on truncated writes

If a POST upload is cancelled after having advertised a content-length,
or a response body is truncated after a content-length, we're not allowed
to send ES because in this case the total body length must exactly match
the advertised value. Till now that's what we were doing, and that was
causing the other side (possibly haproxy) to respond with an RST_STREAM
PROTOCOL_ERROR due to "ES on DATA frame before content-length".

We can behave a bit cleaner here. Let's detect that we haven't sent
everything, and send an RST_STREAM(CANCEL) instead, which is designed
exactly for this purpose.

This patch could be backported to older versions but only a little bit
of exposure to make sure it doesn't wake up a bad behavior somewhere.
It relies on the following previous commit:

   "MINOR: mux-h2: make streams know if they need to send more data"