Rather than hard-coding the default response format as HTML, allow the default to be overridden by an environment variable. For example, given a REST API endpoint that defaults to responding in JSON, you may wish to configure the error messages to be JSON by default as well.
This should fix issue [4039](https://github.com/kubernetes/ingress-nginx/issues/4039). This default backend fails to send the correct `Content-Type` header when it fails to decode the `Accept` request header.
This patch simply forces `text/html` in that specific scenario.