Do not JSON.stringify a plain string (otherwise gets extra quotes)

when doing a `curl -d "x" <url> <etc>` for a 
```
        parameters:
          - name: body
            required: true
            type: string
            paramType: body
```

it becomes `'"x"'` (gets quoted) instead of just `'x'`.
Fix will do the JSON stringify only if the `req.body` is not yet a string