Fix horrible broken mess in 304 handling

Summary:
Sending a response body in a 304 triggers some crazy broken behavior in Safari +
Apache that I never hit during testing. Be spec-compliant.

Test Plan:
Mashed reload a bunch on a .php page with Safari + Apache against localhost.

Reviewed By: tuomaspelkonen
Reviewers: tuomaspelkonen, jungejason, aran, rm
CC: aran, tuomaspelkonen
Differential Revision: 261