Fix infinite recursion in Au formatting

Polymorphism strikes again.  This should have been **obj but let's switch to an
explicit pattern match to be safe.

Fixes #1172.
auto merge of #1174 : kmcallister/servo/au, r=larsbergstrom

Polymorphism strikes again.  This should have been **obj but let's switch to an
explicit pattern match to be safe.

Fixes #1172.

r? @larsbergstrom