chore(view_api): return application/json as content-type for api/v1/form_data endpoint (#24758)

(cherry picked from commit 0631a8086c)

superset/views/api.py

@@ -86,7 +86,7 @@ class Api(BaseSupersetView):
 
         update_time_range(form_data)
 
-        return json.dumps(form_data)
+        return self.json_response(form_data)
 
     @api
     @handle_api_exception

tests/integration_tests/charts/api_tests.py

@@ -1424,6 +1424,20 @@ class TestChartApi(SupersetTestCase, ApiOwnersTestCaseMixin, InsertChartMixin):
         self.assertEqual(rv.status_code, 200)
         self.assertEqual(len(data["result"]), 3)
 
+    def test_query_form_data(self):
+        """
+        Chart API: Test query form data
+        """
+        self.login(username="admin")
+        slice = db.session.query(Slice).first()
+        uri = f"api/v1/form_data/?slice_id={slice.id if slice else None}"
+        rv = self.client.get(uri)
+        data = json.loads(rv.data.decode("utf-8"))
+        self.assertEqual(rv.status_code, 200)
+        self.assertEqual(rv.content_type, "application/json")
+        if slice:
+            self.assertEqual(data["slice_id"], slice.id)
+
     @pytest.mark.usefixtures(
         "load_unicode_dashboard_with_slice",
         "load_energy_table_with_slice",