* GH-3207: RSocket inbound: decode each flux item
Fixes https://github.com/spring-projects/spring-integration/issues/3207
Previously an incoming RSocket Publisher has been decoded as a single unit
leading to extra work on the client side, e.g. a delimiter has to be provided
to treat each payload item as independent
* To have a consistency with Spring Messaging and its `PayloadMethodArgumentResolver`
change an `RSocketInboundGateway` to process inbound payloads as `Flux` and decode
each item independently.
* Change `RSocketDslTests` to remove delimiters and make it consistent with the regular
`RSocketRequester` client
* * Add `decodeFluxAsUnit` option into `RSocketInboundGateway`
* Document the change
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