"Non-hexadecimal digit found" from az keyvault secret download

## Describe the bug

**Command Name**
`az keyvault secret download`

**Errors:**
```
Non-hexadecimal digit found
```

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

- Generate cert using https://github.com/shibayan/keyvault-acmebot
- `az keyvault secret download --vault-name {} --name {} --file cert.pfx --encoding hex`

## Expected Behavior

## Environment Summary
```
Linux-5.4.0-52-generic-x86_64-with-debian-buster-sid
Python 3.6.10
Installer: DEB

azure-cli 2.15.0 *

```
## Additional Context

I don't know what the `hex` encoding would/should be used for, I was just playing around with it while debugging another issue (https://github.com/ansible-collections/ansible.windows/issues/132).