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
```
You can also call `response.raise_for_status()`. Doing so will check that the returned status code indicates success. That is, `raise_for_status` checks that the returned status code is not 4XX or 5XX. Which to use depends on your intent. Do you care about a 200 specifically, or do you just care about success?