Adding `application/health+json` as accepted Content-Type

Hello, love the project idea, but while looking to bring this into an existing codebase, I hit a snag where it was giving a 500 internal server error when using Content-Type `application/health+json`. I would like to follow the rfc health check endpoint spec https://inadarei.github.io/rfc-healthcheck/.

The yaml spec for the endpoint is:

```
/api/health:
get:
summary: Healthcheck to determine health of API.
responses:
200:
description: The request's query parameters.
content:
application/health+json:
schema:
type: object
properties:
status:
type: string
example: "ok"
description:
type: string
example: Description of endpoint.
version:
type: string
example: "0.0.1"
releaseId:
type: string
example: "release-id"
notes:
type: array
items:
type: string
example: [""]
output:
type: string
example: "Output of error"
```

The curl logs are:
```
< HTTP/1.1 500 Internal Server Error
< X-Powered-By: Express
< openapi-cop-source-request: {"method":"GET","path":"/api/health","headers":{"host":"localhost:8888","user-agent":"curl/7.79.1","accept":"*/*"},"query":{}}
< openapi-cop-validation-result: {"request":{"valid":true,"errors":null}}
< Date: Mon, 26 Sep 2022 21:49:33 GMT
< Connection: keep-alive
< Content-Length: 0
```

The server logs with verbose set to 1 are:
```
2022-09-26T21:49:33.007Z openapi-cop:proxy Received server response with status code 200
2022-09-26T21:49:33.008Z openapi-cop:proxy Validation results [GET /api/health]
{
"request": {
"valid": true,
"errors": null
}
}
```

If I change the Content-Type to `application/json` in the response, I get a 200 response, with a `openapi-cop-validation-result` containing `response` and `responseHeaders` objects.

I would be open to contributing if needed.