IResponse.appendHeader(name: string,value: string): IResponse
Appends a response header, preserving any existing values for the same
name rather than replacing them (unlike IResponse.header,
which overwrites). This is the correct way to emit multiple headers of the
same name — most notably several Set-Cookie headers (e.g. an access
cookie plus a refresh cookie, or deleting several cookies at once).
This response, for chaining