Permit processing HTTP/1.1 headers as they arrive

[nsweeting]

I'm currently trying to implement a HTTP-based database driver using mint. One of the features that the DB supports is "progress headers". The headers are continually sent to the client to provide feedback on the query progress. Unfortunately I don't seem to be able to access these until the query is fully executed as all the headers come at once via {:headers, request_ref, headers} - and as noted in the docs "A single :headers response will come after the :status response.". I can confirm that the headers are being sent as the query progresses when the same request is sent with curl.

Is there a reason why mint couldn't provide access to these headers as they arrive instead of accumulating them?

[whatyouhide]

Hi @nsweeting are you using HTTP/1.1 or HTTP/2?

Strike that, it's in the title. Yeah I think we can support this. Want to try sending a PR for this? 🙃