re-apply lost commit to support list_headers and facebook_headers in Message objects

see https://github.com/contextio/contextIO2/commit/fa858abb2bf5c9f4bbcaaf8cfe72b6d985c567ea
allow ContextIO(debug=True) to mean ContextIO(debug="print"), since its what people expect, and not everyone reads the code
response.raw is an exhausted stream. response.content has the actual raw content