title: Adding header to all request with Retrofit 2
body: <p><a href="http://square.github.io/retrofit/" rel="noreferrer">Retrofit 2's documentation</a> says:</p>
  
  <blockquote>
    <p>Headers that need to be added to every request can be specified using an OkHttp interceptor.</p>
  </blockquote>
  
  <p>It can be done easily using the previous version, <a href="https://stackoverflow.com/questions/27213291/how-to-define-a-header-to-all-request-using-retrofit">here's</a> the related QA.</p>
  
  <p>But using retrofit 2, I couldn't find something like <code>setRequestInterceptor</code> or <code>setInterceptor</code> method that can be applied to <code>Retrofit.Builder</code> object.</p>
  
  <p>Also it seems that there's no <code>RequestInterceptor</code> in <a href="http://square.github.io/okhttp/" rel="noreferrer">OkHttp</a> anymore. Retrofit's doc refers us to <a href="https://github.com/square/okhttp/wiki/Interceptors" rel="noreferrer">Interceptor</a> that I didn't quite understand how to use it for this purpose.</p>
  
  <p>How can I do this?</p>