Added null check to AbstractRegionParser to support Spring 4 Groovy BeanBuilder
Minor refactoring to simplify the logic in the org.springframework.data.gemfire.config.AbstractRegionParser.isSubRegion(:Element) method.  In addition, added test cases in the newly created AbstractRegionParserTest class.
Further refactoring to simplify the logic in the AbstractRegionParser.isSubRegion method.