Decode REQUEST_URI to fix issue with URL parameters

Decode REQUEST_URI during dispatch to fix issue with URL parameters.

The following steps produce the issue on Debian 7 with Apache and PHP 5.4, I'm unsure if it affects other setups as well.

Steps to re-create issue:
- Install Concrete5 with "Empty Site" data
- Enable Pretty URLs in the dashboard and make sure that URL rewriting is working
- Set URL_REWRITING_ALL to "true" in the site.php config file

> define('URL_REWRITING_ALL', true);

- Go to the Single Pages dashboard page
- Add a single page with a URL of "non_existent" (or any other invalid single page url)
- The error message will appear URL-encoded

> That%20specified%20path%20doesn't%20appear%20to%20be%20a%20valid%20static%20page.