urldecode()ing of the path

We're doing this now in Drupal 7. The kernel branch isn't yet. After some discussion in #1463656-96: Add a Drupal kernel; leverage HttpFoundation and HttpKernel, I have decided to include it for now after a little refactoring of the path handling code. Basically, I concluded that it's not "wrong" to mess with the path that way because we're already doing horrible things to the path and shoving it off to a separate request attribute, and then mapping based on that. So it's no more of a drupalism than, say, path aliasing. I've left a @todo in the code to revisit the question later if we so choose.

Putting this issue here for record keeping purposes. This should resolve the following related issues:

#1558166: Search Tests
#1557590: Fix UserUserSearchTestCase
#1558346: Fix TaxonomyTermTestCase

Code will be pushed momentarily, so setting this straight to fixed.