bug in unmarshalling of simpleContent

The attached testcase does not work with the current CVS, I get an
JM_EVENT_UNEXPECTED_CHILD_ELEMENT: Unexpected element: 'row'
as it goes into the CellTypeHandler on the endElement call for <row/>.

The attached patch to JAXBGroupSG.java fixes the problem
(moves the test for __handler == null inside the __level > 1
test), and also passes through all the junit tests.