Rails 4.2 and 5 compatibility fix for belongs_to
Rails 4.2 and 5 compatibility fix for belongs_to

Rails 5 adds default presence validation on belongs_to associations:
rails/rails#18233.

Application object need not be present for access token creation in
Resource Owner Password Credentials grant flow.

[fixes #780]
[fixes #787]

mavenlink/doorkeeper,jasl/doorkeeper,EasterAndJay/doorkeeper,dollarshaveclub/doorkeeper,doorkeeper-gem/doorkeeper,moneytree/doorkeeper,dollarshaveclub/doorkeeper,doorkeeper-gem/doorkeeper,mavenlink/doorkeeper,EasterAndJay/doorkeeper,mavenlink/doorkeeper,outstand/doorkeeper,outstand/doorkeeper,nbulaj/doorkeeper,doorkeeper-gem/doorkeeper,jasl/doorkeeper,jasl/doorkeeper,moneytree/doorkeeper,outstand/doorkeeper,nbulaj/doorkeeper