[BUG] Cloning an SSML document turns it into GRXML

* Add a few tests to the test suite to reveal an issue with `GenericElement#clone`.  
  _Warning: Will cause test suite to fail._
* Does NOT include an actual fix for the tests.

Tests added:
* :white_check_mark: cloning SSML documents should match the contents of the original document
* :no_entry: cloning SSML documents should match the class of the original document
* :no_entry: cloning SSML documents should be equal to the original document