Get full URL of images in media library including http://

I’m sure some plugin or your theme modifies your attachment url.
Try to find wp_get_attachment_url hook in your code or simply create your own with high priority this way:

add_filter('wp_get_attachment_url', function($url) {
 return preg_replace("~^//(.+)$~", "https://$1", $url);
}, 999);

or better:

add_filter('wp_get_attachment_url', function($url) {
  return set_url_scheme($url, 'https');
}, 999);

But I recommend you to find the reason of this attachment url behavior first.