Fix SXE->string cast for __toString in inheriting class

Fix SXE->string cast for __toString in inheriting class

This issue was originally hacked around in zend_make_printable_zval.
I've now moved the overridden __toString() handling into the SXE
cast_object handler, so everything invoking that handler
(zend_make_printable_zval, convert_to_string, zval_get_string,
and various other more obscure usages) see the same behavior.