ArgumentCaptor captures caller instead of interface class

Hi Fernando,

Thank you for your tutorial. It is really helpfull for me and got me started writing my first unit test for my android applications. I have one problem though which is described in this stackoverflow post: https://stackoverflow.com/questions/51797291/argumentcaptor-captures-wrong-class

Basically where you have **dummyCallbackArgumentCaptor.getValue().onSuccess(results);**, I get the dummyCaller as a response to **dummyCallbackArgumentCaptor.getValue()**, so naturally i cannot call onSucess(results) on it. As i write these lines I got an idea to actually clone your repository and see if I am getting the tests to pass...