Documents to json show all fields when using only() or exclude()

When converting a QuerySet with an only() or exclude() field to json, the required fields returned in each object are the required ones from the query.

output = Example.objects.only('field_1').to_json()
# Required feilds are returned
# [{'field_1': ...}, ...]

When converting a single document from a query with only paramaters the document containes the fields with default values.

output = Example.objects.only('field_1').first().to_json()
# All fields are returned with default values
# [{'field_1': ..., 'field_2': None, 'field_3': None, ...}, ...]

No files or tests are named. First reproduce the two examples using QuerySet.only() and exclude(), comparing queryset JSON output with a single document's to_json() output. Done means the selected-field behavior is consistent and the resulting JSON matches the fields requested by the query without unexpected default-valued fields.