Removed FIXME in `ScalaSourceFile` (Refactoring)

I thought the only way to get rid of the FIXME was to introduce a
`NullResponse`, which is why I believed it was a good idea to extract a pure
interface from `Response`. As the code demonstrates, it turns out I was wrong in
believing I needed a `NullResponse`.

As a side note, while I think it may still be a good idea to turn `Response`
into a pure interface, I realistically don't see a use-case.  Further, it turns
out that implementing the semantic of a `NullReponse` wouldn't be
straightforward, because the current implementation of `Response` is somewhat
incorrect in my opinion. The issue I see is that a `Response`'s answer shouldn't
change after it is `set`, however this is not enforced in the current
implementation (e.g., one could call `set` and aferward `raise`, and the answer
to `get` would be different depending if it's executed before or after the call
to `raise` - I would consider this a bug). Hence, it currently doesn't make much
sense to turn `Response` into a pure interface.