Fix bug with supporting page url generation

There was an issue with generating URLs for supporting pages that belong
to more than one policy.  The [code in the controller](https://github.com/alphagov/whitehall/blob/build-11392/app/controllers/supporting_pages_controller.rb#L10-L11) specifies the
policy to generate the URL for.  However, this option was being
overridden in the url helper.

This updates the URL helper to only set the policy if one hasn't been
given.

Pivotal: https://www.pivotaltracker.com/story/show/83388244
How about a second assertion where the first policy is passed in to make absolutely sure it's doing the right thing?