If the Server's Shutdown (or SetKeepAlivesEnabled) method was called
while a connection was in a Handler, but after the headers had been
written, the connection was not later closed.

Fixes #9478
Updates #17754 (reverts that workaround)

Change-Id: I65324ab8217373fbb38e12e2b8bffd0a91806072
Reviewed-on: https://go-review.googlesource.com/33141Reviewed-by: Ian Lance Taylor <iant@golang.org>