Strip nils from Model serialization for more compact representation

[jrochkind]

We want to mostly keep unnecessary nils out of serializations, to keep them more compact. This will become even more relevant if we move to a full AttrJson::Attributes implementation -- or something semantically like it -- where by default all attributes are included in serialized hash, even if never set. We don't really need to serialize them all in postgres.

• AttrJson::Model#serializable_hash (and #as_json) now have a new optional strip_attributes keyword argument. Default is false, do same as traditional. true leaves keys for all nil values out of the hash -- but that causes problems for trying to save a nil written on top of another default. So :safely strips nils only for attributes without defaults.

• The AttrJson::Model::Type type class will by default serialize with :safely -- stripping unnecessary nils. But you can change this with a paramter, that you can also pass to AttrJson::Model#to_type. So for instance, when embedding a model somewhere:

     attr_json :some_model, SomeModel.to_type(strip_nils: false)
     attr_json :some_more_model, SomeModel.to_type(strip_nils: true)
  

  If set to to strip_nils: true, the effect is when deserializing a model with an attribute with a default, the default ends up re-applied on top of previously nil values. Kind of making the default applied not just on original creation, but on every de-serialization too.