BATCH-2500: MultiResourceItemWriter with StaxEventItemWriter restarts at position 0

MultiResourceItemWriter delegating to a StaxEventItemWriter doesn't restart properly.
* Changed the open() method in MultiResourceItemWriter by setting the flag 'opened' to true.

Updated PR with code review comments.

spring-batch-infrastructure/src/main/java/org/springframework/batch/item/file/MultiResourceItemWriter.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2006-2007 the original author or authors.
+ * Copyright 2006-2017 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -151,6 +151,7 @@ public class MultiResourceItemWriter<T> extends AbstractItemStreamItemWriter<T>
 		if (executionContext.containsKey(getExecutionContextKey(CURRENT_RESOURCE_ITEM_COUNT))) {
 			// It's a restart
 			delegate.open(executionContext);
+			opened = true;
 		}
 		else {
 			opened = false;