Correct an issue where the wrong "Content-Length" was set for partial content responses

Summary: Ref T8266. Although we compute this correctly above, we ignored it when actually setting the header. Use the computed value to set the "Content-Length" header. This is consistent with the spec/documentation.

Test Plan: Before, some audio (like rain.mp3) was pretty spotty about loading in Safari. It now loads consistently for me locally.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T8266

Differential Revision: https://secure.phabricator.com/D17624