RegionAttributes.offHeap is improperly overridden by RegionFactoryBean.offHeap

Currently, when a user declares a o.a.g.cache.RegionAttributes bean definition in Spring config to configure Off-Heap for an application Region, the attribute is always overridden by the o.s.d.g.RegionFactoryBean.offHeap property, even when the RegionFactoryBean.offHeap property is not explicitly set (defaults to null and therefore Off-Heap defaults to false).

This is problematic since if the user does not explicitly set the RegionFactoryBean.offHeap property, defaulting to null, and say the user declares a RegionAttributes bean definition with Off-Heap configured to true, then the SDG RegionFactoryBean will override this setting and disable Off-Heap due to this incorrect conditional logic.

The user can workaround this bug by always setting the RegionFactoryBean.offHeap property to the desired value, i.e. true or false, which will enable or disable Off-Heap, respectively.