code: protected function buildRequestHeader()
      {
          $bearerToken = $this->bearerToken;
  
          if ($this->bearerToken === null) {
              $bearerToken = $this->getBearerToken();
          }
  
          return array(
              'Authorization: Bearer ' . rawurlencode($bearerToken),
              'Expect:'
          );
      }
docs: Building request HTTP headers
  
  @return array  HTTP headers