protoc-gen-openapi: Enum types that use stringified boolean values like `"TRUE"` are treated as the real boolean values

## env
- protoc-gen-openapi: v0.6.8
- protoc flags: `--openapi_opt naming=proto,enum_type=string`

## proto file

```protobuf
enum Trinary {
NULL = 0;
TRUE = 1;
FALSE = 2;
}

message Condition {
Trinary attached = 1;
}
```

## expected output

```yaml
Condition:
type: object
properties:
attached:
enum:
- 'NULL'
- 'TRUE'
- 'FALSE'
type: string
format: enum
```

## actual output

```yaml
Condition:
type: object
properties:
attached:
enum:
- NULL
- TRUE
- FALSE
type: string
format: enum
```

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I know it's better not to use reserved words, so this is just for usage feedback.