docs: fix ActionEndpoint.action method example
serialize: add support for serialization of sets

Sets are serialized into lists, for two reasons:

  * sets can't hold unhashable objects such as dicts into which
    the Models are usually serialized
  * sets aren't JSON serializable

Also, there is no information loss if serializing into lists,
as long as the receiver doesn't treat the ordering in any special
way.
bump version to 0.0.7