Fix #respond_with_status when headers written or closed

[straight-shoota]

This change makes HTTP::Server::Response#respond_with_status raise when the response is closed or headers have already been written.

ref. #10038 (comment)

This method is also completely broken when headers have already been sent, because the reset causes that flag to disapear and respond_with_status just writes a second response. https://carc.in/#/r/a2ha

This uses the same check_headers method as #10412.

The first commit is just a minor refactoring because the moved spec doesn't belong in the describe group for #respond_with_status.

[bcardiff]

test_alpine worked in this PR 😂

[straight-shoota]

The gods must be pleased then.