Fixes bug described at http://stackoverflow.com/questions/22708773/mvc-sitemap-couldnt-be-found-exception, exception thrown when Mvc.sitemap file is missing, even though EnableSiteMapFile = "false".
Fixed default values for ChangeFrequency and UpdatePriority properties in MvcSiteMapNodeAttribute and MvcSiteMapNode so they always default to "Undefined" if not provided.
Bug: Removed extra call to FindSiteMapNode from within the FindSiteMapNodeFromKey method, since the url table is not likely to contain a URL that corresponds to a key.
Fixes #304, UrlPath.IsAbsoluteUrl method was misidentifying the URL as absolute when an absolute URL was added as a query string parameter, as in the case with ASP.NET error pages.