Added Node.copy(). More Unit Tests.

* Added Node copy method, Element and Attr implementations too.
* Fixed unknown attributes de/serialize () for SerializableObjectModel
* All unknown properties are serialized back again to original file, to
  avoid data lost on de/serialize operation
* Fixed issue with Element content text, using Text node representation
* Added unit test for unknown properties de/serialize
* Added unit test to avoid serialize properties set to null