Do not unconditionally add "//" to absolute URLs and handle authority-less URLs

Navigation: Test authority-less protocols with path.getLocation()
Navigation: Do not assume "//" is always part of an absolute URL
Navigation: Expect a trailing slash when testing path.getLocation()
    
Note: this does indeed represent a fix for gh-6574, but only once a version of
Cordova sporting https://github.com/apache/cordova-wp8/pull/30 is released.
    
Closes gh-6597
Fixes gh-6574
Fixes gh-6599