Fix bug #67704: CURLOPT_POSTFIELDS modify the type of param base php-5.5

curl_setopt will modify the type of param
the bugs is here: https://bugs.php.net/bug.php?id=67704

when the type of param is not string, we use a temp variable， then convert it to string, free the memory of the variable at last

bug fix base php-5.4 is here: https://github.com/php/php-src/pull/924

@laruence in 5.5 that doesn't work if the left operand is `&tmp_current`; I think that was only changed in `master`.