Add saveNode() logic to DummyNode

When saved, the node's original information is saved instead of being saved as a Dummy node.

Address: [MAGN-2586](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-2586), saving file with Revit nodes in Sandbox doesn't corrupt them.

[MAGN-3345](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-3345)