ctx closure issue for v6.3.0
hi , 

i want to use the `app.context.render = render()` by koa-views. but there is  ctx closure issue. below.

https://github.com/queckezz/koa-views/blob/f2dca20ddc83733833bbfcdfce942e5d3ad71ea4/src/index.js#L34

i also found, this issue already fix in master branch, pls help to release a new version thanks.