Remove useless ResponseMock, fix obvious code issues

[alecpl]

I've seen the direction to v5 and that it kinda stalled. So, I thought maybe I could help a bit, using more incremental approach instead of one big change.

So, I run phpstan with level=2 for now. It revealed a lot of obvious code mistakes. A lot of errors were also related to use of ResponseMock class, so I decided to get rid of it. I think it's better without it.

If this approach is gonna be accepted I may spend some more time in this direction.

I did not include a change to phpstan.neon file for now, as it still throws a lot of errors.