/**
  * Builds canonical string from request.
  *
  * @param RequestInterface $request Request to get canonical string from.
  *
  * @return string
  */
 public function getCanonicalString($request)
 {
     $parts = array($request->getHeader('content-type'), $request->getHeader('content-md5'), $request->getPath() . $request->getQuery(), $request->getHeader('date'));
     return join(',', $parts);
 }