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


````
#define ZVAL_COPY_VALUE(z, v)                   \
    do {                                        \
        (z)->value = (v)->value;                \
        Z_TYPE_P(z) = Z_TYPE_P(v);              \
    } while (0)
````
so 
````
ZVAL_COPY_VALUE(&tmp_current, *current)
````
?