Added test for activation key auto_attach

Added a test that updates an activation key's auto_attach field with the
inverse of its current value.
Also added a field to the ActivationKey entity for auto_attach.
```
@Test: Create an activation key, then update the auto_attach ... ok
```
so, in this case i can just use:
```
client.put(
    entities.ActivationKey(id=attrs['id']).path(),
    new_attrs,
    auth=get_server_credentials(),
    verify=False,
).raise_for_status()
```