use cli set Azure service principals tags issue

### **This is autogenerated. Please review and update as needed.**

## Describe the bug

**Command Name**
`az ad sp update`

**Errors:**
```
Refuse str type as a valid iter type.
Traceback (most recent call last):
pip-install-yf22vmg2\knack\knack\cli.py, ln 215, in invoke
cli\core\commands\__init__.py, ln 654, in execute
cli\core\commands\__init__.py, ln 718, in _run_jobs_serially
cli\core\commands\__init__.py, ln 709, in _run_job
cli\command_modules\role\commands.py, ln 69, in graph_err_handler
cli\core\commands\__init__.py, ln 688, in _run_job
cli\core\commands\__init__.py, ln 325, in __call__
...
pip-install-yf22vmg2\msrest\msrest\serialization.py, ln 489, in _serialize
pip-install-yf22vmg2\msrest\msrest\serialization.py, ln 708, in serialize_data
pip-install-yf22vmg2\msrest\msrest\serialization.py, ln 779, in serialize_iter
msrest.exceptions.SerializationError: Refuse str type as a valid iter type.
```

## To Reproduce:
Steps to reproduce the behavior. Note that argument values have been redacted, as they may contain sensitive information.

- _Put any pre-requisite steps here..._
- `az ad sp update --id {} --set {}`

## Expected Behavior

## Environment Summary
```
Windows-10-10.0.18362-SP0
Python 3.6.6
Installer: MSI

azure-cli 2.6.0

```
## Additional Context