v0.2.1

• The object type is now optional. When the objectType key is omitted while an object is deserialized, it is to be assumed that the type of the denormalized object is activity.
• Empty PHP arrays are now dumped as JSON objects instead of empty lists.
• fixed the key of the mbox SHA1 sum property when denormalizing actors
• fixed deserializing incomplete agent objects that are missing the required IRI (the ActorNormalizer wil now throw an exception)
• add a FilterNullValueNormalizer that prevents null values from being serialized
• empty group member lists are not normalized, but the property will be omitted
• ignore nullable result properties when they are not set during normalization and denormalization