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 ]
536c1797202d57efb77bea098e10968ff01602ce universal_scheme
ba000e29ecf3b8df09e0fd363a78cabbe3c2a78f cvs_scheme
1fe48bf82d056f1ece05baccab888357c10c5ab8 r0.1
2e930f84224222ad6514a3c5dc6e00350e199e92 very_cvs
99dc49c5bcfba9d5b412c5fa6d0bf3ba20d68df1 hgkw_standalone_setup
15e8cd7f5295728b089fc8ba236c0f079572fb1d nohook
0c8b7e5c25a6b9a0d2782eaa3748eb546f5a254f archive
0000000000000000000000000000000000000000 universal_scheme
0000000000000000000000000000000000000000 cvs_scheme
0000000000000000000000000000000000000000 r0.1
0000000000000000000000000000000000000000 very_cvs
0000000000000000000000000000000000000000 hgkw_standalone_setup
0000000000000000000000000000000000000000 nohook
0000000000000000000000000000000000000000 archive