I get a rather strong sense of déjà vu looking at lots of spray-http code: it's a pretty good model of what I was already starting to do or what I had in mind a lot of the time.

My own header definitions are pretty clumsy at present; I'm just about up to the stage of improving that with macros now. (I didn't do that to start with so that I could write a few and get a feel for what it would need to be like.)