Fix NSURL#URLPathComponent to use correct component.

This simple patch fixes `- (NSURL *)URLPathComponent` 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
```

Would love to see this merged into master. I created a new `URLTests` file because this test didn't seem to fit in either `RequestTests` or `StringTests`. Open to any feedback you have.