Fix "implode()" argument order in query string construction

Summary: See https://discourse.phabricator-community.org/t/libphutil-exception-when-using-with-newer-php-deprecated-use-of-implode/3159.

Test Plan: Grepped for other calls to implode(), didn't catch any with bad order.

Differential Revision: https://secure.phabricator.com/D20846