request: complete idempotent methods

see rfc2616: http://www.w3.org/Protocols/rfc2616/rfc2616-sec9.html#sec9.1.

Intact idempotent HTTP methods should be:
GET, HEAD, PUT, DELETE, OPTIONS and TRACE

And this should works based on [visionmedia/node-methods](https://github.com/visionmedia/node-methods)? I'm not sure but sure that this should be clear at this module.
/cc @koajs