Fix NSURL path on Windows for UNC paths

[triplef]

Fixes issue on Windows where initializing NSURL with an UNC path like \\server\path\xxx\ and getting back its path adds an extra leading forward slash:

NSURL *url = [NSURL fileURLWithPath:@"\\\\path\\to\\test.txt"];
NSLog(@"url: %@", url); // => file:///%5C%5Cpath%5Cto%5Ctest.txt
NSLog(@"path: %@", url.path); // => /\\path\to\test.txt (extra forward slash)

[rfm]

Great!