Unable to change response for multiple calls on endpoint

Hi, 

using the Responses library, it is possible to change the reponse of a call by calling `add` multiple times.

```
        responses.add('GET', uri, body='{"foo":true}')
        # register different result on second call
        responses.add('GET', uri, body='{"foo":false}')
```

However this seems impossible using `urllib3-mock` or did I do something wrong?