changed filepath vs url detection to make it work with Windows filepaths...

I've encountered a problem with passing file paths to  `addJs()` on Windows platform.

The problem seems to stem from checking for URL schemes via `parse_url` on Windows file paths.

```php
parse_url($filename, PHP_URL_SCHEME);
```

This will return a non-NULL value if  given Windows path (E.g. "C" when given "C:\foo\bar"). Which in turn causes the type check (`FileAsset` vs `HttpAsset`) to erroneously classify these paths as URLs.

I'm also not sure what type of URLs the  second check `strpos($filename, '//:') !== 0` is supposed to catch. Perhaps a typo in the delimiter (`://` instead)?

In any case , I'm proposing a simplified detection mechanism that works across operating systems - identify URLs by an existence check for the "colon slash slash" separator between the protocol and the domain part of an URL.

Hope this makes sense, please review.

Thank you!