Get rid of extra local var that does not add to the logic

There are too many "action name" variables around the process method.
Get rid of unused method in AR Post test model
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]