import { Response } from "https://deno.land/x/deno_koa@v1.0.4/mod.ts";
Append additional header field with value val.
Examples:
this.append('Link', ['<http://localhost/>', '<http://localhost:3000/>']);
this.append('Set-Cookie', 'foo=bar; Path=/; HttpOnly');
this.append('Warning', '199 Miscellaneous warning');