HORNETQ-1370 - Setting String property to null on a Message breaks XMLExporter tool

Conflicts:
	hornetq-tools/src/main/java/org/hornetq/tools/XmlDataImporter.java
	tests/integration-tests/src/test/java/org/hornetq/tests/integration/persistence/XmlImportExportTest.java
HORNETQ-1367 - Allow QueueControlImpl.removeMessage(long) to operate on scheduled messages
Merge pull request #1734 from jbertram/2.3.x

A few commits for EAP