Dynamic request uri

Some authentication libraries need a way to pass through redirect_uri parameters (basically a wildcard of the redirect_uri).

Here's an example:
http://msdn.microsoft.com/en-us/library/microsoft.aspnet.membership.openauth.openauth.requestauthentication(v=vs.100).aspx

The RequestAuthentication method will add querystring attributes like __provider__ and __sid__. 

I've done a match from the start of the URL to prevent a parameter from containing the URL. I'm not sure of the ramifications of allowing in query parameters (I couldn't find much in the OAuth 2 spec), however it doesn't seem like there would be much harm.