Use JSON for all test requests

arkindex/documents/serializers/elements.py

arkindex/documents/tests/test_bulk_children.py

arkindex/documents/tests/test_bulk_classification.py

arkindex/documents/tests/test_bulk_element_transcriptions.py

arkindex/documents/tests/test_bulk_elements.py

arkindex/documents/tests/test_bulk_transcription_entities.py

arkindex/documents/tests/test_bulk_transcriptions.py

arkindex/documents/tests/test_children_elements.py

arkindex/documents/tests/test_classification.py

arkindex/documents/tests/test_create_elements.py

arkindex/documents/tests/test_create_parent_selection.py

arkindex/documents/tests/test_create_transcriptions.py

arkindex/documents/tests/test_edit_transcriptions.py

arkindex/documents/tests/test_element_paths_api.py

arkindex/documents/tests/test_element_type.py

arkindex/documents/tests/test_entities_api.py

arkindex/documents/tests/test_entity_types.py

arkindex/documents/tests/test_metadata.py

arkindex/documents/tests/test_move_element.py

arkindex/documents/tests/test_move_selection.py