[BUG] [GO] ReadOnly fields are marshaled into json

Created by: kke

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
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Description

The JSON/YAML says:

        createdAt:
          description: The date the Secret entity was created.
          format: date-time
          readOnly: true
          type: string

The go struct looks like:

	// The date the Resource entity was created.
	CreatedAt time.Time `json:"createdAt"`

And the MarshalJSON function:

	if true {
		toSerialize["createdAt"] = o.CreatedAt
	}

openapi-generator version

The one that comes from docker run openapitools/openapi-generator-cli