Work around "mb_check_encoding(<stringlike-object>)" warning in particular versions of PHP

Summary: Fixes T13527. Some versions of PHP strictly require that we pass a string value, and reject "stringlike" objects (objects which implement "__toString()").

Test Plan: Ran unit test, although this is somewhat aspirational because my local PHP version isn't affected.

Maniphest Tasks: T13527

Differential Revision: https://secure.phabricator.com/D21193