resteasy-reactive: When using own Content-Types for @Produces entity is always serialized with toString (resteasy-reactive 1.11.0.Beta1)

[holledauer]

Describe the bug
Using @Produces with non-standard content type does not work with resteasy-reactive. It will always serialize with toString.

Expected behavior
It should serialize with the correct serializer depending on the content type (e.g. jackson when Content-Type ends with +json)

Actual behavior
It serializes with toString()

To Reproduce

Steps to reproduce the behavior:

1. Checkout https://github.com/quarkusio/quarkus-quickstarts/tree/development/hibernate-reactive-quickstart
2. Change version to 1.11.0.Beta1
3. Add @Produces("application/vnd.quarkus.fruit-v1+json") to POST method
4. Call curl -H "Content-Type: application/vnd.quarkus.fruit-v1+json" http://localhost:8080/fruits -d'{"name": "apple"}' -v

[ghost]

/cc @FroMage, @geoand, @stuartwdouglas

[geoand]

I have a fix for this but it breaks some of the TCK tests which means I'll need to dig into it more