Make "Range" HTTP header work for Celerity static resource requests

Summary:
Ref T7567. In T8266 I fixed a bunch of obscure "Range" issues, but only for file downloads -- not for Celerity.

Extend all that stuff to Celerity, which is fortunately much easier.

I believe this will fix Conpherence sounds in Safari.

Test Plan:

• Wrote out an HTTP request in a text file with Range: bytes=0-1 and similar, piped it to localhost with cat request.txt | nc localhost 80, saw server return appropriate range responses consistent with file behavior after T8266, which all seems to work.
• Also did that for files to try to make sure I wasn't breaking anything.

Reviewers: chad, amckinley

Reviewed By: chad

Maniphest Tasks: T7567

Differential Revision: https://secure.phabricator.com/D17724