When a URI fails lookup, just 404 if it's POST

Summary: It's rather confusing now since we'll "seamlessly" redirect you to the right URI, but drop the method and parameters.

Test Plan: Hit a bad URI with POST, got 404.

Reviewers: edward, btrahan

Reviewed By: btrahan

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1965