INT-3509 Add Convert Node & DOMSource to Document

JIRA: https://jira.spring.io/browse/INT-3509

**Cherry-pick to 3.0.x and 4.0.x**

Polishing

- Javadoc
- Refactor duplicated code

Conflicts:
	spring-integration-xml/src/test/java/org/springframework/integration/xml/DefaultXmlPayloadConverterTests.java

Resolved.