Fixed typo

`ctx.accept` -> `ctx.accepts`
Merge pull request #53 from ilsken/patch-1

Fixed typo
add charset support to ctx.type=
Merge branch 'master' of github.com:koajs/koa
add ctx.body= setter

this prevents a bunch of redundant checks that middleware may need to
check response length, type etc. the less code floating around based
on our supported response body types the better, giving us more freedom
to change these as needed, and just less error-prone code in general.
fix ctx.body= content-type override check. Thanks @jonathanong
fix wiki link