[Java][Spring] Use better return types for the reactive option

[cbornet]

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• Filed the PR against the correct branch: master, 3.3.x, 4.0.x. Default: master.
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cc @bbdouglas (2017/07) @JFCote (2017/08) @sreeshas (2017/08) @jfiala (2017/08) @lukoyanov (2017/09) @cbornet (2017/09) @jeff9finger (2018/01)

Description of the PR

Wrap ResponseEntity into Mono so it can be filled reactively.
See #571

[wing328]

@cbornet thanks for the PR. Is that a breaking change as the return type has been changed?

[cbornet]

I guess so. I can re-target to 4.0 if you prefer.

[wing328]

Upgrade note

Workaround with the current templates : if you need to set the status code or the headers reactively, you can do so by setting them on the ServerWebExchange in the reactive body. Eg:

    @Override
    public ResponseEntity<Mono<Void>> deletePet(Long petId, String apiKey, ServerWebExchange exchange) {
        Mono<Void> result = Mono.empty()
                .doOnSuccess(it -> exchange.getResponse().setStatusCode(HttpStatus.FORBIDDEN))
                .doOnSuccess(it -> exchange.getResponse().getHeaders().add("foo", "bar"))
                .then();
        return ResponseEntity.status(HttpStatus.OK).body(result);
    }

Ref: #571 (comment)

Credit: @cbornet