serve: don't send any content headers with 304 responses
Fixes HTTP protocol violation introduced in cf5c76017e11. 'hg serve' would show
a stacktrace when loading pages that not had been modified.
There was test coverage for this, but the wrong response headers wasn't shown
and thus not detected.
[ original upstream message ]
# $Id$
include tests/test-keyword tests/test-keyword.out
include MANIFEST.in