Use metadata slice

Take a slice of the metadata if not using composite metadata to allow
reading it multiple times. For composite metadata this is not an isuse
as it is ensured by the underlying RSocket Java API.

spring-messaging/src/main/java/org/springframework/messaging/rsocket/DefaultMetadataExtractor.java

@@ -116,7 +116,7 @@ public class DefaultMetadataExtractor implements MetadataExtractor, MetadataExtr
 			}
 		}
 		else {
-			extractEntry(payload.metadata(), metadataMimeType.toString(), result);
+			extractEntry(payload.metadata().slice(), metadataMimeType.toString(), result);
 		}
 		return result;
 	}