`payload.get('username').lower()` => `AttributeError: 'NoneType' object has no attribute 'lower'`

Just found this on prod
```python
  File "/path/to/site-packages/cornice/service.py", line 493, in wrapper
    response = view_(request)
  File "/path/to/site-packages/patzilla/util/web/identity/service.py", line 56, in identity_auth_handler
    username = payload.get('username').lower()
AttributeError: 'NoneType' object has no attribute 'lower'
```
