remove useless array addition in DrupalWebTestCase::curlInitialize()

aren't we doing the array addition twice here?

      $curl_options = $this->additionalCurlOptions + array(
        CURLOPT_COOKIEJAR => $this->cookieFile,
        CURLOPT_URL => $base_url,
        CURLOPT_FOLLOWLOCATION => FALSE,
        CURLOPT_RETURNTRANSFER => TRUE,
        CURLOPT_SSL_VERIFYPEER => FALSE, // Required to make the tests run on https.
        CURLOPT_SSL_VERIFYHOST => FALSE, // Required to make the tests run on https.
        CURLOPT_HEADERFUNCTION => array(&$this, 'curlHeaderCallback'),
        CURLOPT_USERAGENT => $this->databasePrefix,
      );
      if (isset($this->httpauth_credentials)) {
        $curl_options[CURLOPT_HTTPAUTH] = $this->httpauth_method;
        $curl_options[CURLOPT_USERPWD] = $this->httpauth_credentials;
      }
      curl_setopt_array($this->curlHandle, $this->additionalCurlOptions + $curl_options);