there's a difference between data we can send to spree and the data we
can retrieve.  with this change, we only compare the shared data.

(this a sutty-specific attribute)

Like the `track_inventory` field.

pay what you can is a feature we're developing for spree but currently
is only specific to tienda.sutty.nl

so we can't compare if the product tries to synchronize them and it
would always update