MONGOID-4646 has_one - belongs_to with optional:true incorrectly clears association when assigned itself (#5166)

* MONGOID-4646 fix case when has_one is assigned twice, add new test case that doesn't work

* MONGOID-4646 add pending test for MONGOID-5254

* MONGOID-4646 change condition to equal?

* MONGOID-4646 add more tests and put back self != replacement

* MONGOID-4646 add dup test and clean up tests

* MONGOID-4646 update test for MONGOID-5254 and document expected behavior

* update reference to referenced

Author: neilshweky