add axiosInstance.defaults.headers to an axios call code example

Example 1: axios defaults headers common

var instance = axios.create({
  baseURL: 'https://some-domain.com/api/',
  timeout: 1000,
  headers: {'X-Custom-Header': 'foobar'}
});

Example 2: axios default headers authorization

axios.defaults.headers.common['Authorization'] = AUTH_TOKEN;