fix each() start key

foreach($cursor as $i => $v)
   exit($i); #return 1   -> bug ...

src/BatchQueryResult.php

@@ -107,7 +107,7 @@ public function next()
             if ($this->query->indexBy !== null) {
                 $this->_key = key($this->_batch);
             } elseif (key($this->_batch) !== null) {
-                $this->_key++;
+                $this->_key = $this->_key === null ? 0 : $this->_key + 1;
             } else {
                 $this->_key = null;
             }