Fix boolean casting for nil value

`nil` value is not included in
`ActiveModel::Type::Boolean::FALSE_VALUES` which caused that in Rails 5
the boolean_accessor converted `nil` to `true` instead of `false`.

app/models/service.rb

@@ -206,10 +206,11 @@ class Service < ActiveRecord::Base
     args.each do |arg|
       class_eval %{
         def #{arg}?
+          # '!!' is used because nil or empty string is converted to nil
           if Gitlab.rails5?
-            !ActiveModel::Type::Boolean::FALSE_VALUES.include?(#{arg})
+            !!ActiveRecord::Type::Boolean.new.cast(#{arg})
           else
-            ActiveRecord::ConnectionAdapters::Column::TRUE_VALUES.include?(#{arg})
+            !!ActiveRecord::Type::Boolean.new.type_cast_from_database(#{arg})
           end
         end
       }