fixed the deserialize() error

[muyunli123]

Made the following changes:

• removed all if statements in deserialize() to avoid failing to raise error
• instead, added properties for 4 attributes (product_id, recommended_id, recommendation_type, status)
• made the 4 attributes into hidden variables (e.g., _product_id) in code while keeping the attribute names unchanged in database schema

With the above changes, creating a new recommendation with empty data will raise 400 error