Don't send a close frame on a broken connection.
Don't send a close frame on a broken connection.

This fix is exercised by a test refactor I'm working on in parallel.

biddyweb/websockets,aaugustin/websockets,aaugustin/websockets,andrewyoung1991/websockets,dommert/pywebsockets,aaugustin/websockets,aaugustin/websockets