[!!!][FEATURE] Throw exception for unpersisted changes in Safe Requests
This changeset keeps track if the PersistenceManager has unpersisted changes
and notifies the Developer with a helpful exception if this happens in a
safe request (GET/HEAD). This exception is only thrown in development context
In case you implemented your own Persistence Manager, you must add the new
hasUnpersistedChanges() method, unless you extend the AbstractPersistenceManager.