When implementing AdMob you can define an array of test IDs so that Google knows to serve test ads to these devices, instead of real ads. However, it requires "hashed device IDs". This seems a little vague to me. What ID are they talking about and what hashing method do they expect me to use?

I'm talking about the bit that should go in here:

```
request.testDevices = @[ @"hashed-device-id" ];

```