Don't use mutable defaults

E.g. https://github.com/keycdn/python-keycdn-api/blob/73dd2d6f56f7c9deb896b96a8abfa190f7d29cc5/keycdn/keycdn.py#L34

Instead of:

```python
def get(self, call, params={}):
    ...
```

it needs to be:

```python
def get(self, call, params=None):
    params = params or {}
```

See https://docs.python-guide.org/writing/gotchas/#mutable-default-arguments