Improve accept header validation

[Andreass2]

Feil funnet i DAST testing.
"This endpoint does not reject requests that have invalid Accept headers. While not always a security vulnerability, this is unexpected behavior that could lead to client confusion. In some circumstances, this undefined behaviour may increase the threat surface of the application, by reducing the predictability of the API."

"To ensure that only a valid Accept header is allowed by the endpoint, it is prudent to add a validating step or middleware to the request processing pipeline that handles the requests to a given endpoint."