HStoreDescriptor had an "un-pickable" function at default

Trying to store in cache a model with a DictionaryField raises a PicklingError because the lambda funcion added by django at 'default', removing the key at `__getstate__()` is enough.

```python
PicklingError: Can't pickle <function <lambda> at 0x2752938>: it's not found as djorm_hstore.fields.<lambda> 
```

I was using pylibmc wih django-pylibmc 0.5 and tastypie at heroku.