Prevent overwrtiting of Real GET params

Hi Rowan,

I ran into an issue when using forms for searching. I have an event that is converting GET params from the form into Clean URL Params. It works fine until I try and do a search with the same form from a page that has Clean URL Params already.

As the delegate is fired before the event can process the GET params, the newly added params (if they are the same GET key) are ignored and overwritten.

This may be an edge case, and you may know a better way to do this, but I thought I'd pull request it anyway.

John.