Schema migration is expected to happen after before_save

[rickmak]

For plugin developer, one of the before_save usages is to remove un-wanted attributes to be saved to the database.

For example, conversation have a reference column message. Plugin SDK populate the message as JSON object for easy access. At before_save, the plugin will filter the message away preventing it being saved to DB.

refs: SkygearIO/chat-SDK-JS#65

• Skygear Server Date/Version: 1.1.0
• Application Platform: py-skygear
• Is this a regression?
• Attached logs, screenshots

Expected Results

The message key got filtered out and will not save to DB.

Actual Results

There is schema migration error. Since the passing type (jsonb) is not compatible with the DB type (reference).