Add API Calendar for Return Orders (#4899)

* Add RO to calendar output

* Update title and description to be more descriptive

* Add documentation

* Add simple test for RO calendar

* Lint/syntax

* Lint

* Changes to title, description

InvenTree/order/test_api.py

@@ -2200,3 +2200,15 @@ class ReturnOrderTests(InvenTreeAPITestCase):
         self.assertEqual(deltas['customer'], customer.pk)
         self.assertEqual(deltas['location'], 1)
         self.assertEqual(deltas['returnorder'], rma.pk)
+
+    def test_ro_calendar(self):
+        """Test the calendar export endpoint"""
+
+        # Full test is in test_po_calendar. Since these use the same backend, test only
+        # that the endpoint is available
+        url = reverse('api-po-so-calendar', kwargs={'ordertype': 'return-order'})
+
+        # Test without completed orders
+        response = self.get(url, expected_code=200, format=None)
+        calendar = Calendar.from_ical(response.content)
+        self.assertIsInstance(calendar, Calendar)