private static HttpResponse unsupportedMediaType() {
        return HttpResponse.of(HttpStatus.UNSUPPORTED_MEDIA_TYPE,
                               MediaType.PLAIN_TEXT,
                               "Unsupported media type. Only JSON compatible types and " +
                               "application/graphql are supported.");
    }
This only works reliably if maybeMap is from Json, as maps(objects) in Json
can only have string keys.
graphql-protocol/src/main/java/com/linecorp/armeria/server/graphql/protocol/AbstractGraphqlService.java