Feature Request: get current response headers or unset them alll

[jonathanong]

https://github.com/koajs/koa/blob/fcacb352208115e60fffe145394f904743dfa8c3/lib/context.js#L121 so we don't have to do hacks like this.

Would like at least one of the following:

// get all header keys
res.getHeaderKeys() // => ['content-type', 'content-length']

// unset all headers
res.unsetHeaders()

I think (from reading issues a while ago) that all the headers are stored in node as [ [<key>, <value>] ] pairs. i don't mind making that public, either.

[jonathanong]

oh sorry, didn't read #1873 thoroughly enough