Fixed bad upvalue for MOAILuaClass::_extendSingleton()

InitLuaSingletonClass() tries to create a C closure from the
_extendSingleton() method with the userdata and class table as
upvalues. Even after halfnelson's fix, however, it instead used two
copies of the class table as upvalues. As a result, _extendSingleton()
always received a NULL-pointer as first upvalue (because a table cannot
be converted to userdata).
Merge pull request #864 from DanielSWolf/InitLuaSingletonClass-bug

Fixed bad upvalue for MOAILuaClass::_extendSingleton()