We don't need to cast the value a second time in uniqueness validations

Part of the larger refactoring to remove type casting from Arel. Since
we've already cast the value a few lines above, we don't need to re-cast
it later. We can inform Arel of this by wrapping it in an
`Arel::Nodes::Quoted`, which will no longer be required in Rails 5.1

activerecord/lib/active_record/validations/uniqueness.rb

@@ -65,6 +65,7 @@ module ActiveRecord
           value = value.to_s[0, column.limit]
         end
 
+        value = Arel::Nodes::Quoted.new(value)
         if !options[:case_sensitive] && value && column.text?
           # will use SQL LOWER function before comparison, unless it detects a case insensitive collation
           klass.connection.case_insensitive_comparison(table, attribute, column, value)