Allow JsonNode auto-convert into ArrayNode if duplicates found (for XML)

[cowtowncoder]

(note: follow-up for FasterXML/jackson-dataformat-xml#229)

So, XML presents unique problem for JsonNode support in that there is a possibility of seeing what looks like duplicate properties: this is common in XML since sequences of elements are legal, and do not need to be unique unlike JSON properties.
One consequence of this is that currently (as of 2.11) use of JsonNode with XML is limited.

But it seems relatively simple to allow special mode in which such duplicates would be used to "auto-convert" existing entry for property into ArrayNode (if it isn't already one) to allow better support.

[cowtowncoder]

Actually, I think I got a better idea: by adding StreamReadCapability.DUPLICATE_PROPERTIES (true for XML, false for other formats), it should be possible to simply "make it work" as expected, without JsonNodeConfig.