Documentation: response.set(fields) overwrites previous header fields? 
Under Response (koa response object), there's `response.set(fields)`. The text reads "Set several response header fields with an object:," with an example. However, it's not clear if setting these fields via object means that the header becomes ONLY those fields, or whether passed-in-object extends the previous header. It's also not clear what the behavior is if the fields are identical to current header values. 

It would be helpful to clarify these questions in the documentation. I still haven't figured the answer out. 

https://github.com/koajs/koa/blob/master/docs/api/response.md#responsesetfields