COCO Segmentations produces invalid Json

[agibsonccc]

Describe the bug A clear and concise description of what the bug is. Currently segmentations produce json arrays that aren't delimited with commas properly. An example entry:

  }{
      "segmentation":[
        [

To Reproduce Steps to reproduce the behavior:

1. Annotate any dataset with polygons
2. Download as COCO Expected behavior A clear and concise description of what you expected to happen. Valid JSON is output from the COCO downloader. Screenshots If applicable, add screenshots to help explain your problem.

Desktop (please complete the following information):

• OS: [e.g. Windows, Ubuntu] Windows
• Browser [e.g. chrome, firefox] Chrome
• Version [e.g. 1.0.0] 1.0..0

Additional context Add any other context about the problem here.