Fix AssistedBuilder injection memory leak

Long story short - following example code will leak memory when used together with current version of Injector because each ``AssistedBuilder(...)``-generated object will cause creation of "new" ``BindingKey`` on injection. Patch attached.

```python

class A(object): pass

injector = Injector()

while True:
    injector.get(AssistedBuilder(A))
```