Relax check on default data MimeType

If there is more than one non-basic codec (e.g. CBOR and JSON)
RSocketRequester.Builder takes the mime type of the first one rather
than giving up. It is a valid scenario (JSON for server responding to
browser, and CBOR for client talking to server) and it is the default
situation in Boot, and after all the point here is to pick some default
as best as we can with the worst possible outcome being a server
refusing the connection if it doesn't support the mime type. Beyond
that applications can set the dataMimeType on the builder explicitly.

To match that change this commit also ensures RSocketMessageHandler
rejects proactively data mime types it does not support at the point
of accepting a connection.

spring-messaging/src/test/java/org/springframework/messaging/rsocket/DefaultRSocketRequesterBuilderTests.java

@@ -46,7 +46,6 @@ import org.springframework.util.MimeTypeUtils;
 import org.springframework.util.ReflectionUtils;
 
 import static org.assertj.core.api.Assertions.assertThat;
-import static org.assertj.core.api.Assertions.assertThatThrownBy;
 import static org.mockito.ArgumentMatchers.any;
 import static org.mockito.ArgumentMatchers.anyInt;
 import static org.mockito.BDDMockito.given;
@@ -131,22 +130,6 @@ public class DefaultRSocketRequesterBuilderTests {
 				.isEqualTo(MimeTypeUtils.APPLICATION_JSON);
 	}
 
-	@Test
-	public void defaultDataMimeTypeWithMultipleCustomDecoderRegitered() {
-		RSocketStrategies strategies = RSocketStrategies.builder()
-				.decoder(new TestJsonDecoder(MimeTypeUtils.APPLICATION_JSON))
-				.decoder(new TestJsonDecoder(MimeTypeUtils.APPLICATION_XML))
-				.build();
-
-		assertThatThrownBy(() ->
-				RSocketRequester
-						.builder()
-						.rsocketStrategies(strategies)
-						.connect(this.transport)
-						.block())
-				.hasMessageContaining("Cannot select default data MimeType");
-	}
-
 	@Test
 	public void dataMimeTypeSet() {
 		RSocketRequester requester = RSocketRequester.builder()