The PropertySerializer contains :id, :name and :presentation attributes
which leads to JSON objects like `{"id": 1, "name": "foo",
"presentation": "foo"}`. Because of this, we don't care about object
identity, just about their presentation attribute.
By not having to treat producer properties differently we can filter and
fetch all properties from DB without having to process it with Ruby and
with a single query.
See this commit for more details: 18e5b98f5c (diff-b0846898827183f530c113ad7b83b8ea)
Also:
- remove shipping method restriction on calculators to inherit from Spree::Shipping::ShippingCalculator so that OFN customized calculators keep working
- add shipping method serializer spec to test serialization of all shipping methods configured