Validating a document in-memory

1. Call hasFeature("????", "3.0") to verify that this is supported

2. Load the document in the usual way

3. Load the ASModel

4. Cast the Document to a DocumentAS

5. Attach the ASModel the DocumentAS using the setAS() method

6. Invoke the DocumentAS's validate() method

7. If the Document is not valid, then a ASException is thrown with the code VALIDATION_ERR