* Use properly `cache()` for the `Mono` in the `ClientRSocketConnector`
* Don't require a `publisherElementTypeExpression` in the
`RSocketOutboundGateway`: the target element type is determined
automatically in the `DefaultRequestSpec.toResponseSpec()`
* SF has deprecated `MediaType.APPLICATION_JSON_UTF8`: fix all its usage
in favor of recommendations to use the `MediaType.APPLICATION_JSON` instead
* Also fix plain `application/json;charset=UTF-8` literals to use
`MediaType.APPLICATION_JSON` instead
e2b661845e