Better check if the page is in dashboard

Detail: http://www.concrete5.org/developers/bugs/5-6-3-1/using-dashboard-as-the-first-word-in-a-page-url-disables-admined/
I think we should pass the Page object to helper in this file. Jason says "If you navigate to the same page using cID code it works." at bug tracker, but this is also a bug. These are same page with both style (collection path or cID=xx), so `inDashboard()` should return same values.