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[1] 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.

[1] https://github.com/alphagov/whitehall/blob/build-11392/app/controllers/supporting_pages_controller.rb#L10-L11
Merge pull request #1873 from alphagov/fix_supporting_page_url_bug

Fix bug with supporting page url generation

https://www.pivotaltracker.com/story/show/83388244