XML support

This is a feature request.

We have the following OpenAPI document:

# ... metadata
paths:
  /api/path:
    post:
      summary: ...
      description: ...
      operationId: operation
      requestBody:
        required: true
        content:
          application/json:
            schema:
              $ref: '#/components/schemas/Model'
          application/xml:
            schema:
              $ref: '#/components/schemas/Model'
      responses:
        # ...

The model contains an xml key to describe the XML tag which is used to instantiate the model.

components:
  schemas:
    Model:
      xml:
        name: model
      type: object
      properties:
        # ...

Then, we fire an XML request, but this fails the validation

# ./spec/controllers/api/...'
# ------------------
# --- Caused by: ---
# OpenAPIParser::ValidateError:
#   #/components/schemas/Model expected object, but received String: <?xml version="1.0" encoding="UTF-8"?>
#   <model>
#     <property>94cf1162-0897-42f9-9dcb-9bd9abe7aa17</property>
#   </model>
#   ./spec/controllers/api/...

The XML is returned correctly, and matches the OpenAPI specification. The request validator interprets the XML response (application/xml content type) as a String.

Start with the request validator exercised by spec/controllers/api/... and trace how the application/xml request body is parsed before schema validation. Done means the XML body is validated as the Model object described by the OpenAPI document, while the existing XML response behavior remains correct.