Remove route attribute from RouteValueDictionary

When mvcSiteMapNode is populated with 'route' attribute, it selects the
correct route but adds it as a query param as well, e.g. '?route=bfs'.
This commit ensures route query param is removed only if route is
present.
Merge pull request #122 from beresovskiy/master

Remove 'route' param from query string when routeName is supplied