C API: Provide a way to get the raw response header bytes

[seanmonstar]

What if... the C API provided an option to ask for the original header data, as a single unaltered buffer? It'd be cheap in hyper, just a ref count bump on that buffer, and in curl you wouldn't need to re-stitch together the headers into lines, or have a discrepancy of whether it was \r\n or just \n, and the order would what was sent.

It could just be an option set on either hyper_request * or hyper_clientconn_options * (perhaps the conn since you only need to set it once per connection). And then you could fetch it from a hyper_response *:

struct hyper_buf *hyper_response_headers_raw(struct hyper_response *resp);