Fix double escaping in phutil_tag

Summary:
I wasn't able to reproduce the "recursion detected" in real web request but I saw lots of 1073741824 refcounts in debug_zval_dump() of $object.
I'm not sure how that happens.

Test Plan: D4807#4

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2432

Differential Revision: https://secure.phabricator.com/D4839