Avoid implict User::__toString in SpecialContributions.php

Follow-Up: Ie07001e75a9732f727930f8f367bba8feb395b3e
Change-Id: Ibe24bbe6d1cd7a0148353f1aa9cf6f11109dc1de
Merge "Avoid implict User::__toString in SpecialContributions.php"