docs: def response_headers():
      """Returns a set of response headers from the query string.
      ---
      tags:
        - Response inspection
      parameters:
        - in: query
          name: freeform
          explode: true
          allowEmptyValue: true
          schema:
            type: object
            additionalProperties:
              type: string
          style: form
      produces:
        - application/json
      responses:
        200:
          description: Response headers
      """
      # Pending swaggerUI update
      # https://github.com/swagger-api/swagger-ui/issues/3850
      headers = MultiDict(request.args.items(multi=True))
      response = jsonify(list(headers.lists()))
  
      while True:
          original_data = response.data
          d = {}
          for key in response.headers.keys():
              value = response.headers.get_all(key)
              if len(value) == 1:
                  value = value[0]
              d[key] = value
          response = jsonify(d)
          for key, value in headers.items(multi=True):
              response.headers.add(key, value)
          response_has_changed = response.data != original_data
          if not response_has_changed:
              break
      return response