Oh okay. Didn't realise content headers were set differently. Should I apply the same sort of change at line 64 as well?

I might need to rework this block a little bit because the way it's written at the moment doesn't allow setting a null/empty header to cancel a header that's been added on the interface or method. TBH, I'm not sure if that's a likely use case, but it feels like it might be counter-intuitive to only allow additive overrides. What do you think?