Protocol Relative links on Windows

Hi Simon.

Thanks for this plugin – it’s part of my standard install.

I’ve found an obscure issue. I’ve noticed that if I enqueue an external script using a protocol relative link (ie. starting with ‘//’ rather than hard-coding ‘https://’) that works fine on Linux, but on Windows its interpreted as a file path, and, for me, takes 20 seconds to fetch it.

Easily fixed in my code, but might be good to add a check for that and add the protocol? Something like this in modification_time method works:

if ( strpos( $src, '//' ) === 0 ) {
	$protocol = is_ssl() ? 'https:' : 'http:';
	$src = $protocol . $src;
	}

Thanks!