[Bug] shutdown sends GOAWAY with reason internal error about gun

[essen]

Both shutdown and {shutdown, _} should result in a NO_ERROR.

[zuiderkwast]

Protocol:closing/4 is only called from gun:closing/2 which is internal. gun:closing/2 is only ever called with 'shutdown' and 'owner_down' from within the same module. Thus, I believe handling of any other reason is dead code.

This 'closing' reason is not the same as the exit reason used elsewhere, for example in disconnect/2.

[essen]

Yes as far as the code is currently implemented only shutdown needs to be handled. But in the future it would be better to handle sys:terminate or EXIT signals properly and in that case both shutdown and the tuple should result in a NO_ERROR. But for now a smaller patch is fine.

[zuiderkwast]

To handle supervisor shutting down gun connections, EXIT and sys:terminate, we would need to use trap exit. I think it would be good to have, but it's a larger change. I made a PR with only a minimal fix.