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.
hg debuginstall
$ hg debuginstall
checking encoding (ascii)...
checking Python lib (*lib*)... (glob)
checking installed modules (*mercurial)... (glob)
checking templates (*mercurial?templates)... (glob)
checking commit editor...
checking username...
no problems detected
hg debuginstall with no username
$ HGUSER= hg debuginstall
checking encoding (ascii)...
checking Python lib (*lib*)... (glob)
checking installed modules (*mercurial)... (glob)
checking templates (*mercurial?templates)... (glob)
checking commit editor...
checking username...
no username supplied (see "hg help config")
(specify a username in your configuration file)
1 problems detected, please check your install!
[1]