updated request.md and response.md

As per our discussion on https://github.com/koajs/koa/issues/342 , I have updated the response.md and request.md to reflect the actual API as the current one confuses beginners .Also on line 93 in response.md, I have changed  'res.body`  to 'this.body` based on line 166 in response.js. @jonathanong