RESTEasy Reactive: Content type incorrect for Qute templates

[FroMage]

Describe the bug

efecf5e introduced a bug because the TemplateResponseFilter used to set the Content Type:

            if (mediaType != null) {
                responseContext.setEntity(r, null, mediaType);
            } else {
                responseContext.setEntity(r);
            }

While the new behaviour in TemplateResponseUniHandler does not:

        requestContext.setResult(createUni(requestContext, (TemplateInstance) result, engine));
    }

    private Uni<String> createUni(ResteasyReactiveRequestContext requestContext, TemplateInstance result, Engine engine) {
        setSelectedVariant(result, requestContext.getRequest(),
                requestContext.getHttpHeaders().getAcceptableLanguages());
        return toUni(result, engine);
    }

As a result, Qute HTML templates now get their content type set to text/plain; charset=utf8 which is incorrect. A bit scary that we don't have a test for this too…

[quarkus-bot]

/cc @geoand, @mkouba, @stuartwdouglas

[geoand]

Seems like you have a fix really 😎.

Mind providing a PR?

[geoand]

Also, it would be great to have a simple way to reproduce this

[geoand]

@FroMage could you test #25945?