Maximum header length?
Hi,

I appear to be hitting an issue with maximum header lengths. Koa 2.0.0 / Node 6.5.0 / Ubuntu 16.04 x64. There is no error logged and no response is sent to the client, looks like the connection is just closed.

[Repro](https://github.com/mhingston/koa-issue)

Edit: Forgot to mention, I'm using nginx as a reverse proxy, I suspect the issue is its buffer sizes. Will close this if I can determine if nginx is the source here.
