code: @Override
      public Map<String, String> getHeaders() throws AuthFailureError {
          Map<String, String> headers = super.getHeaders();
          if (!mRequestHeaders.isEmpty()) {
              if (headers.isEmpty()) {
                  return mRequestHeaders;
              } else {
                  headers.putAll(mRequestHeaders);
              }
          }
          return headers;
      }
docs: get request header