Cookie headers are being overwritten by later setCookie headers

[cmorten]

Issue

Setup:

• Deno Version: 1.9.2
• Opine Version: 1.3.2

Details

Attempting to upgrade superdeno to support Deno ^1.9.0 noticing that the cookie tests are failing because Opine is not appending cookies set in headers, instead incorrectly overwriting them...

const app = opine();

    app.get("/", (req, res) => {
      res.cookie({ name: "foo", value: "bar" });
      res.cookie({ name: "user", value: "deno" });
      res.set("Set-Cookie", "fizz=buzz");

      console.log(res.headers); // Showing only `"set-cookie": "fizz=buzz",`. Missing the other 2 cookies.

      res.set("X-Tested-With", "SuperDeno");
      res.type("application/json");
      res.end();
    });

    superdeno(app)
      .get("/")
      .expect(function (res) {
        expect(res.headers).toEqual({
          "content-length": "0",
          "x-powered-by": "Opine",
          "set-cookie": "foo=bar; Path=/, user=deno; Path=/, fizz=buzz", // Seeing `fizz=buzz` instead and fails ❌
          "x-tested-with": "SuperDeno",
          "content-type": "application/json; charset=utf-8",
        });
      })
      .expect(200, done);

[cmorten]

This is now the expected behaviour of set following the adjustment of the underlying Header class in Deno to match the web specification. To append cookies instead use .append() not .set().