Allow magic methods to be called recursively.

This creates the possibility of infinite-loop recursion if used messily, but
fixes an Undefined Property bug occuring when a getter tried to re-enter itself
with a different property name. See Issues:

- #254 (https://github.com/facebook/hiphop-php/issues/254)
- #348 (https://github.com/facebook/hiphop-php/issues/384)
- #657 (https://github.com/facebook/hiphop-php/issues/657).