Filter only possibly-tainted keys from superglobals

Summary: Ensures that weird behavior from filter_input_array does not remove keys from superglobals. Should fix T3677.

Test Plan:
Checked that $_SERVER contained same number of keys before and after
filtering, and that those affected by the original bug continue to be filtered
correctly.

Reviewers: epriestley, btrahan

Reviewed By: epriestley

CC: zorfling, aran, Korvin, wez

Maniphest Tasks: T3677

Differential Revision: https://secure.phabricator.com/D6680