trait_get does not differentiate traits from gettable non-traits attributes (e.g. instance methods)

Not sure if this was the intended behaviour of `trait_get`:
```
>>> class Foo(HasTraits):
>>> mapping1 = Dict(Str(), Str())

>>> f = Foo()

>>> f.trait_get(["add_trait", "mapping1"])
{'add_trait': >,
'mapping1': {}}
```