Change `XmlWriteFeature.UNWRAP_ROOT_OBJECT_NODE` default to `true`

[pjfanning]

https://github.com/FasterXML/jackson-dataformat-xml/blob/master/src/main/java/tools/jackson/dataformat/xml/XmlWriteFeature.java#L45-L61

Comment says that we may be changing the default in 3.0

[cowtowncoder]

Good catch @pjfanning ! Will do PR for change.

[cowtowncoder]

Went ahead and changed 3 other defaults for #727, #728 and #729.

Can still re-consider if need be, based on feedback for 3.0.0-rc1.