Extracted XmlNamespacesTest
Do not produce XML declaration for toString/toPrettyString
Removed debug code
Simplified toDocument to not be namespace-aware
Refactored XmlNamespaceTest: test the element and attribute together in each scenario
TODO notes
Unit tests for XmlSchemaValidator
Added XmlDocumentGenerator.toInputStream()
Refactored: extracted IOUtil
Removed XmlDocumentGenerator.toXml()

It was not namespace-aware and it also exposed Java's XML APIs a bit too much