Unable to 'Unshare' a shared link via update.

- [X] I have checked that the [SDK documentation][sdk-docs] doesn't solve my issue.
- [X] I have checked that the [API documentation][api-docs] doesn't solve my issue.
- [X] I have searched the [Box Developer Forums][dev-forums] and my issue isn't already reported (or if it has been reported, I have attached a link to it, for reference).
- [X] I have searched [Issues in this repo][github-repo] and my issue isn't already reported.

### Description of the Issue
We are unable to issue the following command via the CLI in order to remove a shared link expiry date
```
box files:shared-links:update --unshared-at="null"
Cannot parse date format "null"
```
```
box files:shared-links:update --unshared-at=null
Cannot parse date format "null"
```

This however works.
`box request https://api.box.com/2.0/files/ -X PUT --body '{"shared_link": {"unshared_at": null}}'`

### Steps to Reproduce
Issue commands as above

### Expected Behavior
allow the unshared-at to use null as an option

### Error Message, Including Stack Trace

### Screenshots

### Versions Used
Box CLI: 2.8
Operating System: macOS Catalina

[sdk-docs]: ./doc
[api-docs]: https://developer.box.com/docs
[dev-forums]: https://community.box.com/t5/Platform-and-Development-Forum/bd-p/DeveloperForum
[github-repo]: https://github.com/box/box-java-sdk/search?type=Issues