Bug: validations do not run on POST with invalid content type header

[bart-degreed]

To repro, start the demo project and POST to http://localhost:1479/api/v1/tags with the next body:

{
    "data": {
        "type": "tags",
        "attributes": {
            "name": "John"
        }
    }
}

and header Content-Type: "application/json". This succeeds without error.

Changing the Content-Type header to "application/vnd.api+json" results in a validation error, as expected.