Add Service.data attribute validator to drop additional fields
Add Service.data attribute validator to drop additional fields

Uses SQLAlchemy validators to remove any additional fields when
the attribute is assigned.

(http://docs.sqlalchemy.org/en/rel_0_9/orm/mapped_attributes.html#simple-validators)

alphagov/digitalmarketplace-api,alphagov/digitalmarketplace-api,alphagov/digitalmarketplace-api