no error when adding `sops` key to YAML file with `sops edit`

I've used `sops edit file.yaml` to create a YAML file like: ```yaml # file.yaml sops: "something" ``` After saving the change, `sops` is unable to decrypt the file: it complains of two `sops` keys in the YAML. Please add a check for reserved key names to SOPS, and prevent saving a file with an illegal key name (`sops`).