Edited Issue: Setting route value in route conventions instead of adding [756]

This fix is try to workaround tracing issue: http://aspnetwebstack.codeplex.com/workitem/755<br />As route convention will be run twice, it will cause duplicate keys are inserted to route values collection and result in duplicate key exception thrown.<br /><br />The workaround is to set those values instead of adding. So even it's already there, it won't cause exception and the logic doesn't change.