added resetWithinGroupOrderBy method

Author: oohnoitz

src/SphinxQL.php

@@ -1402,30 +1402,42 @@ public function reset()
     public function resetOrderBy()
     {
         $this->order_by = array();
+
         return $this;
     }
 
     public function resetWhere()
     {
         $this->where = array();
+
         return $this;
     }
 
     public function resetMatch()
     {
         $this->match = array();
+
         return $this;
     }
 
     public function resetGroupBy()
     {
         $this->group_by = array();
+
+        return $this;
+    }
+
+    public function resetWithinGroupOrderBy()
+    {
+        $this->within_group_order_by = array();
+
         return $this;
     }
 
     public function resetOptions()
     {
         $this->options = array();
+        
         return $this;
     }
 }