Make the list of streams final in CompositeItemStream

Resolves BATCH-1969

Author: fmbenhassine

spring-batch-infrastructure/src/main/java/org/springframework/batch/item/support/CompositeItemStream.java

@@ -31,15 +31,15 @@
  */
 public class CompositeItemStream implements ItemStream {
 
-	private List<ItemStream> streams = new ArrayList<>();
+	private final List<ItemStream> streams = new ArrayList<>();
 
 	/**
 	 * Public setter for the {@link ItemStream}s.
 	 * 
 	 * @param streams array of {@link ItemStream}.
 	 */
 	public void setStreams(ItemStream[] streams) {
-		this.streams = Arrays.asList(streams);
+		this.streams.addAll(Arrays.asList(streams));
 	}
 
 	/**