Fix a bug getting 0

Fix a bug that prevented one to get an attribute of an Item which has 0 value. (Or any value that bool() casts to False.) I'm not quite sure if this change has no side effect.

In the old version when self.get(name) returns the Value 0.0, __getattr__ also checked self.kwargs.get(name), which returned None. Where as if I used .get(name) directly the expected 0.0 was returned. I think this was also intended in __getattr__.