added the assert.

it is implicitely called. see:

```python
10477 ◯ : ipython                                                                                                                             
Python 2.7.5+ (default, Aug  4 2013, 10:07:17) 
Type "copyright", "credits" or "license" for more information.
  1 class BaseConfig(object):
  2     def __init__(self):
  3         print "base init"
  4 
  5 
  6 class ServiceConfig(BaseConfig):
  7     pass
  8 
  9 
 10 class FooConfig(ServiceConfig):
 11 
 12     def __init__(self):
 13         ServiceConfig.__init__(self)
In [5]: f = FooConfig()
base init
```