…Invalid JSON payload error (#439)

`auth.create_custom_token()` results in an error:

```
Failed to sign custom token. Error calling the IAM signBytes API:{ (...)
  "error": {
    "code": 400,
    "message": "Invalid JSON payload received. Unknown name \"{\"bytesToSign\": \"...\"}\": Cannot bind query parameter. Field '{\"bytesToSign\": \"...\"}' could not be found in request message.",
    "status": "INVALID_ARGUMENT",
    "details": [
      {
        "@type": "type.googleapis.com/google.rpc.BadRequest",
        "fieldViolations": [
          {
            "description": "Invalid JSON payload received. Unknown name \"{\"bytesToSign\": \"...\"}\": Cannot bind query parameter. Field '{\"bytesToSign\": \"...\"}' could not be found in request message."
          }
        ]
      }
    ]
  }
}
```

I have confirmed setting `'Content-Type': 'application/json'` header resolves the problem.

* chore: updated CHANGELOG.md [ci skip]

* chore: updated setup.cfg [ci skip]

* chore: updated setup.py

Co-authored-by: release-please[bot] <55107282+release-please[bot]@users.noreply.github.com>