Add support for multipart/form-data request

With this PR, jdoc can display `Content-Type` and body of a request when `link.encType` is specified as `multipart/form-data`.

The dump of a request body is performed by `Rack`. 

If we have nested field names in definition of link schema, such as 
```yaml
schema:
  properties:
    key1:
      key2:
        "$ref": "file"
```

, then  it will be displayed as `[key1][key2]`, using Rack (and Rails) convention.