Align with Framework changes to RSocket routing and new message/x.rsocket.routing.v0 requests

[wilkinsona]

Latest Framework snapshots are causing RSocketWebSocketNettyRouteProviderTests to fail with the following:

java.lang.IllegalArgumentException: No encoder for java.lang.String
	at org.springframework.messaging.rsocket.RSocketStrategies.encoder(RSocketStrategies.java:61)
	at org.springframework.messaging.rsocket.DefaultRSocketRequester$DefaultRequestSpec.encodeMetadata(DefaultRSocketRequester.java:274)
	at org.springframework.messaging.rsocket.DefaultRSocketRequester$DefaultRequestSpec.lambda$getMetadata$6(DefaultRSocketRequester.java:252)

The request that can't be encoded has a MIME type of message/x.rsocket.routing.v0 that was added to Framework in this commit.

[wilkinsona]

@bclozel @rstoyanchev Can you please confirm that this was the right change to make? Given that all mime types or just text/plain are the two options for CharSequenceEncoder it seemed reasonable. It's also the encoder that Framework's tests use so hopefully I'm not too far wide of the mark.

[rstoyanchev]

That's a reasonable change for now and in general, although you should be able to switch back to the stricter "text/plain" when spring-projects/spring-framework#23137 is resolved.

[philwebb]

I've raised #17146 to make sure we remember to switch back.