Error if a set-cookie header is set on the response

[CrowdHailer]

cowboy treats the set-cookie header as a special case

https://github.com/ninenines/cowboy/blob/master/src/cowboy_http.erl#L1218

[CrowdHailer]

There are a few issues that show people have tripped over this difference before.

• https://github.com/ninenines/cowboy/issues/1124
• https://github.com/ninenines/cowboy/issues/1329

It's worth noting that the type spec for headers in cowboy does not correctly encompass this. https://github.com/ninenines/cowboy/blob/master/src/cowboy.erl#L34 Potentially a PR to fix that would be valuable.

To wrap cowboy will require treating this header as a special case