http2: fix subsequent end calls to not throw

Calling Http2ServerResponse.end multiple times should never
cause the code to throw an error, subsequent calls should
instead return false. Fix behaviour to match http1.

Fixes: https://github.com/nodejs/node/issues/15385
PR-URL: https://github.com/nodejs/node/pull/15414


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>