Make custom consumers easier to write

[joeatwork]

A Consumer-based workaround for #41 will likely require some cut-and-paste code, or referring to private Consumer methods, or both. The whole point of the Consumer architecture is to allow people to write their own custom consumers, and while many of them will ultimately delegate to our consumer, some (like the requests-based backend) will not. Writing a consumer for a different HTTP stack should be a half-page of code at most, and shouldn't duplicate code that is already in the library.