Support exceptions for clean url redirects

Hi, I was working with a payment module for Drupal Commerce and it turned out that the third-party provider required a specific router path to be non-clean. Having enabled the option to redirect all non-clean urls to clean urls, the request returned 301 which resulted in failure for the payment to get through. I think the best solution for this kind of problem is to allow for exceptions to the rule so I made a patch to achieve this functionality.