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
Wouldn't this all best be served by simply doing:

```javascript
if (thing.isObject(value)) {
    req.body = value = JSON.stringify(value);
}
```

This should cover the case where express converts to an object.

Although actually we likely should do something like `querystring.stringify` instead.