Fix URLWithPath to use correct component.

This simple patch fixes `- (NSURL *)URLWithPath` to correctly set the path component.

Before the patch you'd get:

``` objc
NSLog(@"%@", [[NSURL URLWithString:@"http://host.local"] URLWithPath:@"/test"]);
// http://host.local
```

After the patch you get:

``` objc
NSLog(@"%@", [[NSURL URLWithString:@"http://host.local"] URLWithPath:@"/test"]);
// http://local.host/test
```

I created a new `URLTests` file because this test didn't seem to fit in either `RequestTests` or `StringTests`. Would love to see this merged into master. Open to any feedback you have.