Resolved issue serializing deleted keys to the server after call to Clear()

[devmonkey22]

Regardless of if SerializedRawData found and removed the key, mark the key as deleted to handle when Clear() is called, which clears all SerializedRawData immediately, so then keys never really get deleted at the server.

[devmonkey22]

@welegan Can you confirm if this satisfies your intent between the logic in RedisSessionStateItemCollection.Clear() which also clears the SerializedRawData collection too; and the RedisSessionStateItemCollection.GetChangedObjectsEnumerator() called during RedisSessionStateStoreProvider.SerializeToRedis ?