fix: set Content-Type header in the request to signBlob API to avoid Invalid JSON payload error

[kimamula]

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.

[kimamula]

I think this is a serious bug which makes it impossible to create a custom token using a service account ID with Firebase Admin Python SDK.
The feature is crucial for our app which is to be released next week.
Please tell me if there is something I can do for this fix to be released.

[kimamula]

I ended up using private key file.

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