ISPN-2609 Infinispan SpringCache throws java.lang.NullPointerException: Null values are not supported!

Add support for storing null values in and retrieving null values from a
SpringCache. Use solution provided by Roland Csupor as a starting point.

* Introduce NullValueSupportingSpringCacheWrapper which wraps a vanilla
SpringCache and adds support for storing and retrieving null values.
* Make *CacheManagers and *CacheManagerFactoryBean aware of optional
support for null values.