Fix AxiosRequestHeaders & AxiosHeaders types;

[DigitalBrainJS]

• Added missed iterator declaration for AxiosHeaders interface;
  Now iteration over AxiosHeaders is valid for TS as well; Closes axios 1.2.3 causes an AxiosHeader error #5476;

const headers = new AxiosHeaders({foo: "bar"})

for (const [header, value] of headers) {
  console.log(header, value);
}

• Improved AxiosRequestHeaders type to be able to handle assignment of an AxiosHeaders instance;

  const headers:AxiosRequestHeaders = new AxiosHeaders({x:1});

  headers.y = 2;

  headers.get('x');

  const requestInterceptorId: number = axios.interceptors.request.use(
      async (config) => {
        config.headers.Accept ="foo1";
        config.headers.setAccept("foo2");
        config.headers = new AxiosHeaders({x:1});
        config.headers.foo = "1";
        config.headers.set('bar', '2');
        // config.headers = {...config.headers, myHeader: "myValue"}; // throws type error in TS, but works in JS runtime, possible workaround:
         config.headers.set({myHeader: "myValue"})
        // config.headers = {myHeader: "myValue"}; // throws type error in TS, but works in JS runtime, possible workaround:
         config.headers = new AxiosHeaders({myHeader: "myValue"});
         // OR
         config.headers = {...config.headers} as AxiosRequestHeaders;
        // since Axios will automatically convert plain object headers to an AxiosHeaders instance after interceptors and transforms calls
        return config;
      },
      (error: any) => Promise.reject(error)
  );

[nguyenvuhoang]

     config.headers = {...config.headers} as AxiosRequestHeaders;