Fix where_unscoping on 4.0, 4.1, 4.2

[chewi]

Believe it or not, basic unscope functionality was broken. I don't know how but with Squeel in play, where_values somehow becomes a nested array. Flattening it first fixes the issue.

I have adjusted the 4.1 override to use super so that less code is duplicated. This isn't possible for 4.0. I have added a new override for 4.2 because where_values should not be mutated in that version.