fix: set Content-Type header in the request to signBlob API to avoid 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.2 files changed
