Alternate transfer encoding (e.g. content-length) needs work and testing

[carlbrown]

In working on writing a framework on top of this one, I've discovered that I quite often want to send a payload that has a fixed length (e.g. a JSON Data produced by Codable), and the code paths to do that (e.g. in HTTPStreamingParser.swift when isChunked == false) are hard to use and poorly tested.

I need to fix that.