http2: fix closedCode NaN, increase test coverage

[kFinish](code) can be triggered from a 'finish' event (for example
when calling response.end) which does not pass code. That tries to
set closedCode to undefined resulting in NaN closedCode instead of
NGHTTP2_NO_ERROR. Check for code !== undefined before setting.
Adds tests for closed and closedCode.

PR-URL: https://github.com/nodejs/node/pull/15154


Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>