Fix data type of filtered /data JSON response

[ukanga]

Changes / Features implemented

Using the json.loads() to return field values as default type. Django switched from this approach here.

Steps taken to verify this change does what is intended

GET /api/v1/data/{form pk}.json?fields=["_id", "_xform_id_string"]
[{"_id": 1, "__xform_id_string": "id_string"}, {"_id": 2, "__xform_id_string": "id_string"}, ..]

instead of

GET /api/v1/data/{form pk}.json?fields=["_id", "_xform_id_string"]
[{"_id": "1", "__xform_id_string": "\"id_string\""}, {"_id": "2", "__xform_id_string": "\"id_string\""}, ..]

Side effects of implementing this change

Before submitting this PR for review, please make sure you have:

• Included tests
• Updated documentation - n/a

Closes #2254