code: public function setHeader($header, $value)
      {
          if (strlen($header) < 1) {
              throw new Exception('Header must be a string.');
          }
          $this->customHeaders[$header] = $value;
          return $this;
      }
docs: Sets a custom header to be sent together with the request.
  Throws an exception if header's name is not at least 1 char long.
  
  @param string $header
  @param string $value
  @return $this
  @throws Exception