Update go-net to 20151229. Instead of boring commitlogs, my summary of
changes is:

- http2 package replaces spdy. New interactive HTTP2 debugger, h2i.
- New context/ctxhttp for context-aware HTTP request handlers.
- New xsrftoken package for generating and checking XSRF tokens.
- Improved HTML5-capable HTML parser.

(bsiegert)