prevent throw error for undefined text

src/shapes/Text.js

@@ -150,7 +150,7 @@
             context.fillStrokeShape(this);
         },
         setText: function(text) {
-            var str = Konva.Util._isString(text) ? text : text.toString();
+            var str = Konva.Util._isString(text) ? text : (text || '').toString();
             this._setAttr(TEXT, str);
             return this;
         },

test/unit/shapes/Text-test.js

@@ -13,6 +13,20 @@ suite('Text', function(){
         layer.draw();
     });
 
+    // ======================================================
+    test('text with undefined text property should not throw an error', function() {
+        var stage = addStage();
+        var layer = new Konva.Layer();
+
+        stage.add(layer);
+        var text = new Konva.Text({text: undefined});
+
+        layer.add(text);
+        layer.draw();
+
+        assert.equal(text.getWidth(), 0);
+    });
+
     test('add text with shadows', function() {
         var stage = addStage();
         var layer = new Konva.Layer();