Fix duplicate set-cookie headers

[jpillora]

Currently, this will result in 2 set-cookie headers. One in Headers and one in Cookies.

	http.SetCookie(w, &http.Cookie{
		Name:  "foo",
		Value: "bar",
	})

Without this fix, the included test fails on assert.Equal(t, 1, len(e.Headers)) (has 2, Content-Type + Set-Cookie)

Also @wolfeidau @tj the tests are passing, though like in #39, I'm guessing this will fail with AWS Build/Semaphore - are these build processes broken? Maybe we can just disable them and just use Actions to run the tests.

[wolfeidau]

@jpillora yeah i would love to sort this out but I got removed from the org recently 😞 cc @tj