Sort headers intelligently

[davecheney]

From #16

I thought about this previously and have two suggestions.

1. http headers are not sorted, they probably get unsorted more in this version compared to the python version because of the way maps work in Go vs whatever curl is doing.
2. if we are going to sort headers, it shouldn't just be plain alpha sort, it should
   • a. put Host: first
   • b. sort headers first into their categories, hop by hop and end to end (https://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html#sec13.5.1), then sort those alphabetically.

I know this is more work, but I think this is only worth doing if it's done to this degree of precision.

[davecheney]

Note to implementors, ignore point 2a, this is the response, Host is not part of the response headers. This was my mistake.