HTTP::absoluteUrls() doesn’t touch srcset URLs

[lozcalver]

Example: when URLs in emails are automatically rewritten to be absolute.

Not sure what (if anything) can be done about this. It’s a static method, so can’t use Injector. I’m not even sure if the regex will replace more than one URL at a time anyway:

https://github.com/silverstripe/silverstripe-framework/blob/229672a49d83d12a1d1f1546ba3c1d9b771b5835/src/Control/HTTP.php#L165-L178

May need a patch for core to make this issue fixable.