Use "QUERY_STRING", not "REQUEST_URI", to parse raw request parameters

Summary:
Fixes T13260. "QUERY_STRING" and "REQUEST_URI" are similar for our purposes here, but our nginx documentation tells you to pass "QUERY_STRING" and doesn't tell you to pass "REQUEST_URI". We also use "QUERY_STRING" in a couple of other places already, and already have a setup check for it.

Use "QUERY_STRING" instead of "REQUEST_URI".

Test Plan: Visited /oauth/google/?a=b, got redirected with parameters preserved.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13260

Differential Revision: https://secure.phabricator.com/D20227