Fixes: mockito#2644

Fixes issues around vararg handling for the following methods in `AdditionalAnswers`:
 * `returnsFirstArg`
 * `returnsSecondArg`
 * `returnsLastArg`
 * `returnsArgAt`
 * `answer`
 * `answerVoid`

These methods were not correctly handling varargs. For example,

```java
doAnswer(answerVoid(
      (VoidAnswer2<String, Object[]>) logger::info
   )).when(mock)
      .info(any(), (Object[]) any());

mock.info("Some message with {} {} {}", "three", "parameters", "");
```

Would previously have resulted in a `ClassCastException` being thrown from the `mock.info` call.  This was because the `answerVoid` method was not taking into account that the second parameter was a varargs parameter and was attempting to pass the second actual argument `"three"`, rather than the second _raw_ argument `["three", "parameters", ""]`.

avoiding having to cast to `Invocation`