Fix Rails 5.2 incompatibility#5

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flackou wants to merge 3 commits intomwlang:masterfrom

flackou:fix-rails5.2

Commits on May 2, 2018

Change Rails locked version to 5.2
Julien Lestavel
committed
Replacing the global AR config `belongs_to_required_by_default = false` by individual `optional: true` on every `belongs_to` association
Julien Lestavel
committed
Add a test on optional `belongs_to` association : a Post can be saved without parent, it will use the column's default value 0
Julien Lestavel
committed