Add ToXmlGenerator.Feature.UNWRAP_ROOT_OBJECT_NODE (to avoid root ObjectNode wrapper element)

[cowtowncoder]

Jackson 2.12 improved handling of JsonNode on reader-side (f.ex see #205, #403 and #405).
But when trying to serialize JsonNode we still get an unnecessary <ObjectNode> wrapper.
This should be peeled away; although for backwards-compatibility we may want to put this behind a ToXmlGenerator.Feature (default setting to be debated).

It may also make sense consider "unwrapping" of other types, depending on how implementation turns out (may end up with 2 features, one for JsonNode, other for the rest -- if others are even feasible).