Raises the CacheItemExpiring event.
Raising an event invokes the event handler through a delegate.
The OnCacheItemExpiring method also allows derived classes to handle the event without attaching a delegate. This is the preferred technique for handling the event in a derived class.
Notes to Inheritors: When overriding OnCacheItemExpiring in a derived class, be sure to call the base class's OnCacheItemExpiring method so that registered delegates receive the event.