DNN-4785 Do not lowercase registered URLs

The client resource manager's RegisterScript (and RegisterStyleSheet) methods call ToLowerInvariant on the file paths, but some URLs are case sensitive (e.g. the Google Maps API requires passing a case-sensitive API key on the query-string, and WebResource.axd URLs have a case-sensitive hash).

This change removes the lower-casing.

I'm assuming that the lower-casing was done to aid de-duplication, but given that more requested scripts are using the DnnJsInclude and DnnCssInclude controls, I'd have to hope that the de-duplication mechanism is able to do case-insensitive comparisons...