Resolves https://github.com/spring-projects/spring-integration/issues/2197
Sometime we would like to avoid some XML definition features (e.g. loading of the DTD).
The `XmlValidatingMessageSelector` doesn't allow to do that easily because it doesn't
rely on the `Document` creation via `DocumentBuilderFactory`, but directly
manipulates with `Source`
* Change `DefaultXmlPayloadConverter.convertToSource()` to fallback to the `convertToDocument()`
for non-Document payloads.
This way we can configure `DefaultXmlPayloadConverter` with custom `DocumentBuilderFactory` and
bypass DTD loading, for example
* Expose `xml-converter` option for the `<int-xml:validating-filter>` component
d79e70c6df