[Regression 4.0 -> 4.1] Put back Relation#join method as a delegate to Array

This is a regression 4.0 -> 4.1 fix.
In 4.1.0 `Relation#join` is delegated to `Arel::SelectManager`.
In 4.0 series it is delegated to `Array#join`

This patch puts back the behaviour of 4.0