OpenAPI - Add support for application/xml

Describe the bug
I have an openapi spec I'm loading into ZAP, that looks like this for 1 path:

/rest/sqli-00/BenchmarkTest00568/send:
  post:
    operationId: dopost_568
    requestBody:
      description: Form POST parameters in request body
      required: true
      content:
        application/xml:
          schema:
            type: object
            properties:
              name:
                type: string
              address:
                type: string
    responses:
      default:
        description: default response
        content:
          application/xml: {}

But ZAP is sending:

POST https://localhost:8443/benchmark/rest/sqli-00/BenchmarkTest00568/send HTTP/1.1
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:82.0) Gecko/20100101 Firefox/82.0
Pragma: no-cache
Cache-Control: no-cache
Content-Length: 57
Accept: application/xml
Host: localhost:8443

{"name":"ZAP","address":"John Doe"}

As you can see, even though the request content type is: application/xml ZAP is sending application/json instead.

To Reproduce
Load an API spec with application/xml as the request content type.

Expected behavior
It should send XML formatted data in the request, not json.

Software versions

• ZAP: 2.10.0
• Add-on: OpenAPI plugin v20

Start by loading the reported OpenAPI specification into ZAP and reproducing the request generated for an application/xml request body. Trace the OpenAPI add-on's request generation from the content-type selection through body serialization. Done means the request uses an XML content type and sends XML-formatted data rather than the shown JSON body.