Set default value to ActiveModel::Validator initializer attribute

Makes it easier to test its objects.
adding small mention one single line blocks in contributing to rails
[ci skip]
Merge pull request #8849 from mattdbridges/contributing_to_rails

Adding mention about single-line blocks in Contributing to Rails [ci skip]
Merge pull request #8848 from Irio/master

Set default value to ActiveModel::Validator initializer attribute