Merge two Headers object. The first Headers always takes precedence. When fields conflict, the first Headers takes precedence if it is a singleton field. If it is a list-based field and not empty, it is appended to the first Headers field. Invalid field names and field values are ignored. No destructive operation is performed on the arguments and returns a new Headers object.

import { mergeHeaders } from "https://deno.land/x/http_utils@$VERSION/mod.ts";
import { assertEquals } from "https://deno.land/std@$VERSION/testing/asserts.ts";

assertEquals(
  mergeHeaders(
    new Headers({ accept: "text/html" }),
    new Headers({ accept: "application/json", "content-type": "text/plain" }),
  ),
  new Headers({
    accept: "text/html, application/json",
    "content-type": "text/plain",
  }),
);
assertEquals(
  mergeHeaders(
    new Headers({ origin: "http://test.test" }),
    new Headers({ origin: "http://example.test" }),
  ),
  new Headers({ origin: "http://test.test" }),
);
// origin is singleton field