PIVOT-887: Make changes to Spinner and BXMLSerializer such that using the DefaultProperty of Spinner (which is "spinnerData") in a BXML file will work correctly.

Normally if the original value is a Sequence (as it always will be for the spinner data) the child is added to the sequence.  But using an ImmutableList enables BXMLSerializer to catch the UnsupportedOperationException and do a beanAdapter.put instead (which results in a call to "setSpinnerData"), which is the correct thing to do.

git-svn-id: https://svn.apache.org/repos/asf/pivot/trunk@1428650 13f79535-47bb-0310-9956-ffa450edef68