GEOS-6374 GetMapXmlReader always uses default style for layergroup layers

As code from an old version of GetMapKvpRequestReader.java was copied into GetMapXmlReader.java I followed suite and copied/looked at the newer version applying fixes for layergroup.
Added new test file as I could find no existing test for GetMapXmlReader.

I bet the overlapping functionality could be separated into a common class. Although, I'm not comfortable with that since there are no regression tests for that refactor.