Validation fails for OAS3 headers that are $refs in encoding object

[alasdairhurst]

Given the following request body, this is disallowed:

      description: The URL body
      content:
        'text/plain':
          schema:
            $ref: '#/components/schemas/URL'
          examples:
            foo:
              $ref: '#/components/examples/foo'
            xml:
              $ref: '#/components/examples/xml'
          encoding:
            url:
              headers:
                id:
                  $ref: '#/components/headers/id'

The spec allows items in the headers map to be reference objects.
https://github.com/OAI/OpenAPI-Specification/blob/master/versions/3.0.0.md#encoding-object

[alasdairhurst]

This is actually an issue with the official OAS schema!
OAI/OpenAPI-Specification#2157