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
```
Can you call `create_json` here instead of `create`? The `create` method is currently a thin wrapper around `create_json`, but at some point in the future that will change. In the future, `create_json` will continue to return a dict of attributes as shown here, and `create` will return an object with normal attributes.