I was just working on this one when I saw that
looks like this on my Pi-hole:
[i] Target: https://raw.githubusercontent.com/StevenBlack/hosts/master/hosts
[✓] Status: No changes detected
[i] Analyzed 58750 domains
Can you confirm the server raw.githubusercontent.com is still not sending the header for you?
I will likely still add this check you requested here, however, I will need some new test lists now 
edit @yubiuser can you recommend one for me for testing? I picked a few random ones (not githubusercontent) from https://v.firebog.net/hosts/lists.php?type=tick but all of them seem to be able to deliver the header just fine:
