Handle "multipart/form-data" correctly even if we get the data

Summary: Fixes T3673. Supposedly we won't get any data in this case, but it seems we sometimes do. See discussion in task.

Test Plan: Used `var_dump()`, etc., to verify we short circuit out of "multipart/form-data" posts regardless of the presence of input data.

Reviewers: nmalcolm, btrahan

Reviewed By: nmalcolm

CC: aran

Maniphest Tasks: T3673

Differential Revision: https://secure.phabricator.com/D6670