http2: fix EOF handling on uploads with auth negotiation

http2: fix EOF handling on uploads with auth negotiation

- doing a POST with `--digest` does an override on the initial request
  with `Content-Length: 0`, but the http2 filter was unaware of that
  and expected the originally request body. It did therefore not
  send a final DATA frame with EOF flag to the server.
- The fix overrides any initial notion of post size when the `done_send`
  event is triggered by the transfer loop, leading to the EOF that
  is necessary.
- refs #11194. The fault did not happen in testing, as Apache httpd
  never tries to read the request body of the initial request,
  sends the 401 reply and closes the stream. The server used in the
  reported issue however tried to read the EOF and timed out on the
  request.

Reported-by: Aleksander Mazur
Fixes #11194
Cloes #11200