import { Response } from "https://deno.land/x/opine@2.3.4/src/response.ts";
Append additional header field with value val.
Value can be either a string or an array of string.
Example:
res.append('Set-Cookie', 'foo=bar; Path=/; HttpOnly'); res.append('Warning', '199 Miscellaneous warning'); res.append("cache-control", ["public", "max-age=604800", "immutable"]);