client.changeThreadColor doesn't work with gradients

## Description of the problem
The color thread doesn't change when a gradient is specified: (`TICKLE_ME_PINK`, `MALACHITE`, `RUBY`, `DARK_TANGERINE` and `BRIGHT_TURQUOISE`). The other colors do work.

## Code to reproduce
```py
client.changeThreadColor(ThreadColor.BRIGHT_TURQUOISE, id_of_the_thread)
```

## Traceback
```
DEBUG:urllib3.connectionpool:https://www.facebook.com:443 "POST /messaging/save_thread_color/?source=thread_settings&dpr=1 HTTP/1.1" 200 None
DEBUG:client:{'__ar': 1, 'payload': None, 'bootloadable': {}, 'ixData': {}, 'bxData': {}, 'gkxData': {}, 'qexData': {}, 'lid': '6786391857354966290'}
```

## Environment information
- Python 3.7.3
- `fbchat` 1.9.6