Align Reactive RestEasy default mime types

[michalszynkiewicz]

At the moment, resteasy reactive content type is set to plaintext for string return type and json otherwise by default.
The rest client on the other hand is aligned to the MicroProfile specification and always uses application/json by default.

This needs to be aligned, probably it would be best to change the client to behave like server and have a switch so that it can be compliant with the spec.