[Fix] Session quantity fix

Row exists in session, sum up the session quantity and the new quantity given to the add method.

src/Cart.php

@@ -99,7 +99,7 @@ class Cart
         $content = $this->getContent();
 
         if ($content->has($cartItem->rowId)) {
-            $cartItem->qty++;
+            $cartItem->qty += $content->get($cartItem->rowId)->qty;
         }
 
         $content->put($cartItem->rowId, $cartItem);
@@ -500,4 +500,4 @@ class Cart
     {
         return config('cart.database.table', 'shoppingcart');
     }
-}
+}