Added a check to not allow duplicated question marks in uris

URLs that are initially set up to have a query string, and later parameters are added to the request, build an URI with two question (?) marks. The library should handle those situations. 

Maybe this is agains the REST spec or even agains RestSharp premisses, but nevertheless it seems to be to be a fairly fualusible situation of everyday developer, specially those using RestSharp as a general HTTP library.