Remove to_s from reflection.type in add_constraints

The instance var is already saved as a string in the initialization
method of AssociationReflection.

See https://github.com/rails/rails/blob/master/activerecord/lib/active_record/reflection.rb#L273
Merge pull request #16550 from eileencodes/remove-unnecessary-to_s-from-add_constraints-method

Remove to_s from reflection.type in add_constraints