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
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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
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