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
Merge pull request #14978 from bogdan/relation-join

[Regression 4.0 -> 4.1] Put back Relation#join method as a delegate to Array
Improve CHANGELOG entry [ci skip]