change res.type= to always default charset. Closes #252
Merge branch 'master' of ssh://github.com/koajs/koa
Release 0.5.3
refactor res.status= and res.body=

remove res.body=null in res.status=. it will cause cycle calls in same
cases.
move strip headers into respond, make sure all no content response won't
set these headers.
fix default status set bug in this.body=null.