Detach principal from thing when it is deleted

[Sytten]

This fixes #2514

[coveralls]

Coverage decreased (-0.002%) to 93.573% when pulling 2d5e2e9 on Sytten:fix/delete_iot_principal_thing into 3834c74 on spulec:master.

[mikegrima]

In your tests, can you add some logic to confirm that the mutations occurred? I.e. can you confirm if the principals were actually detached?

[Sytten]

I can, but it will throw on the last call if the mutations did not occur anyway.

[mikegrima]

I see so if the code was not there then it would fail anyway?

[Sytten]

Yes, but I added a check. Its a good practice.

[mikegrima]

Will merge when tests pass.