We don't need to perform type casting on the STI condition

We will always have the correct type for this query, so no casting is
needed. We inform Arel that we already have the right type by wrapping
it in an `Arel::Nodes::Quoted` (which we will no longer need to do in
Rails 5.1)

activerecord/lib/active_record/inheritance.rb

@@ -193,6 +193,7 @@ def find_sti_class(type_name)
       def type_condition(table = arel_table)
         sti_column = table[inheritance_column]
         sti_names  = ([self] + descendants).map(&:sti_name)
+        sti_names.map! { |v| Arel::Nodes::Quoted.new(v) } # FIXME: Remove this when type casting in Arel is removed (5.1)
 
         sti_column.in(sti_names)
       end