setdefault not work

It's interesting that setdefault does not call `__setitem__`

```python

from easydict import EasyDict

a = EasyDict()
a['a'] = a
print(a['a'])
print(a.a)
a.setdefault('b', 2)
print(a['b'])
print(a.b) # <--- AttributeError: 'EasyDict' object has no attribute 'b'

```