Just compiled Node from the master branch and tried to use koa. It failed because the property values in this.res._headers are now arrays instead of strings.

$ node
> http = require('http');
> resp = new http.ServerResponse({});
> resp.setHeader('Content-Type', 'foo');
> result = resp._headers

In Node 7.x, result is { 'content-type': 'foo' }

In Node master (pre-8.0), result is { 'content-type': [ 'Content-Type', 'foo' ] }

The change was made here: nodejs/node@c00e4ad#diff-286202fdbdd74ede6f5f5334b6176b5cR406 (Node PR: nodejs/node#10558)

@mscdex noticed that it broke express and provided a fix https://github.com/jshttp/fresh/pull/20/files Something similar would be needed for koa.

I'll open this issue to make sure this issue is solved for koa in some way.