COCO Segmentations produces invalid Json

**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:
```json
}{
"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**
Add any other context about the problem here.