If a header has been set as a default, there does not appear to be any way to skip it on an individual request. Setting `null` or `undefined` doesn't do anything.

test/specs/headers.spec.js
function testHeaderValue(headers, key, val) {
  var found = false;
  for (var k in headers) {
    if (k.toLowerCase() === key.toLowerCase()) {
      found = true;
      expect(headers[k]).toEqual(val);
      break;
    }
  }
  if (!found) {
    if (typeof val === 'undefined') {
      expect(headers.hasOwnProperty(key)).toEqual(false);
    } else {
      throw new Error(key + ' was not found in headers');