Removes Modification to Filter Structure

* Impacts API when using filters. Has a documentation impact

```python
client['Account'].getVirtualGuests(
    filter={'virtualGuests': {'domain': 'example.com'}})
```
would become
```python
client['Account'].getVirtualGuests(
    filter={'virtualGuests': {'domain': {'operation': 'example.com'}})}
```

This more closely matches the API and allows for more complex operations like date range filtering and the 'in' operation.