Kotlin: make Item class equivalent to C# and Java

Kotlin/src/main/kotlin/com/gildedrose/Item.kt

@@ -1,3 +1,7 @@
 package com.gildedrose
 
-data class Item(var name: String, var sellIn: Int, var quality: Int)
+open class Item(var name: String, var sellIn: Int, var quality: Int) {
+    override fun toString(): String {
+        return this.name + ", " + this.sellIn + ", " + this.quality
+    }
+}