Update on I18n_JSON field fails to remove keys if provided keys already exist in object

[johnatawnclementawn]

When performing an update on an I18n_JSON field, the custom as_sql method seems to only check that the keys to update exist in the set of keys that already exist in the object, rather than checking that the keys provided are the set of keys that exist in the object.

This fails when trying to remove keys during an update, e.g.
original_obj = {"label":"...", "defaultValue":"...", "options":[], "placeholder":"...", "i18n_properties":["placeholder"]}
updated_obj={"label":"...", "placeholder":"...", "i18n_properties":["placeholder"]}