title: Set custom headers to all volley requests
body: <p>Overriding getHeaders() works to add customs headers if necessary to the requests.
  But what if X header is always needed to be set? for example an static auth token.</p>
  
  <pre><code>JsonObjectRequest req = new JsonObjectRequest(URL, new JSONObject(params),
              new Response.Listener&lt;JSONObject&gt;() {
                  @Override
                  public void onResponse(JSONObject response) {
                      setFollowButton(item);
                  }
              }, null){
          @Override
          public Map&lt;String, String&gt; getHeaders() throws AuthFailureError {
              HashMap&lt;String, String&gt; headers = new HashMap&lt;String, String&gt;();
              headers.put("Authorization", "Token XXXXXXXXXXXX");
              return headers;
          }
  };
  
  Application.getInstance().addToRequestQueue(req);
  </code></pre>
  
  <p>Can be the header added to the request sent to the addToRequestQueue method instead overwriting it every time I write a request? </p>