fixed bug different type command returns

bug detail
i.g.
````php
$ret = $redis->multi()
             ->set("stringkey", "val")
             ->lRange("stringkey", 0, 0)
             ->exec();
var_dump($ret);
````
expect:
````
array(2) {
  [0]=>
  bool(true)
  [1]=>
  bool(false)
}
````
actual:
````
array(1) {
  [0]=>
  bool(true)
}
````