Can't export polygons as COCO dataset

[zhiltsov-max]

Version 39eb9c5

Exporting the task with polygons as COCO dataset can lead to the error:

Traceback (most recent call last):
File "/usr/local/lib/python3.5/dist-packages/rq/worker.py", line 812, in perform_job
rv = job.perform()
File "/usr/local/lib/python3.5/dist-packages/rq/job.py", line 588, in perform
self._result = self._execute()
File "/usr/local/lib/python3.5/dist-packages/rq/job.py", line 594, in _execute
return self.func(self.args, *self.kwargs)
File "/home/django/cvat/apps/dataset_manager/task.py", line 273, in export_project
server_url=server_url)
File "/home/django/cvat/apps/dataset_manager/task.py", line 152, in export
self._dataset.export_project(converter=converter, save_dir=save_dir)
File "/home/django/datumaro/datumaro/components/project.py", line 689, in export_project
converter(dataset, save_dir)
File "/home/django/datumaro/datumaro/plugins/coco_format/converter.py", line 607, in call
converter.convert()
File "/home/django/datumaro/datumaro/plugins/coco_format/converter.py", line 555, in convert
task_conv.save_annotations(item)
File "/home/django/datumaro/datumaro/plugins/coco_format/converter.py", line 362, in save_annotations
elem.update(self.convert_points_object(points))
File "/home/django/datumaro/datumaro/plugins/coco_format/converter.py", line 385, in convert_points_object
visibility = ann.visibility
AttributeError: 'Polygon' object has no attribute 'visibility'