Why do not return drawn elements after changeMode ('draw_line_string ')

**mapbox-gl-js version**:
**mapbox-gl-draw version**:

### Steps to Trigger Behavior

1.
2.
3.

### Expected Behavior

### Actual Behavior

The issue provides no mapbox-gl-js or mapbox-gl-draw versions, reproduction steps, expected behavior, actual behavior, files, or tests. Start by obtaining a minimal reproduction for changeMode('draw_line_string') and compare the returned drawn elements with the expected result; done means the behavior is reproduced, explained, and covered by a regression test.

Written by the indexing model from the issue text.