refactor to use ctx.responseLength
add docs
remove negotiator usage

it has a bug, but already in koa.
https://github.com/federomero/negotiator/issues/10
refactor threshold check

this should be fine right?
remove semis

to match :D sorry haha
remove "error" handling for stream

koa does it
add handling of "error" for source stream

koa cant grab these :(