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
}
  }
}
describe('headers', function () {
  beforeEach(function () {
    jasmine.Ajax.install();
  });
  afterEach(function () {
    jasmine.Ajax.uninstall();
  });
  it('should default common headers', function (done) {
    var headers = axios.defaults.headers.common;
    axios('/foo');
    getAjaxRequest().then(function (request) {
      for (var key in headers) {
        if (headers.hasOwnProperty(key)) {
          expect(request.requestHeaders[key]).toEqual(headers[key]);
        }
      }
      done();
    });
  });
  it('should add extra headers for post', function (done) {
    var headers = axios.defaults.headers.common;
    axios.post('/foo', 'fizz=buzz');
    getAjaxRequest().then(function (request) {
      for (var key in headers) {
        if (headers.hasOwnProperty(key)) {
          expect(request.requestHeaders[key]).toEqual(headers[key]);
        }