BATCH-483: putting null in an ExecutionContext no longer throws an exception. Also added a unit test.

spring-batch-infrastructure/src/test/java/org/springframework/batch/item/ExecutionContextTests.java

@@ -95,4 +95,9 @@ public class ExecutionContextTests extends TestCase{
 		}
 	}
 	
+	public void testPutNull(){
+		//putting null should work
+		context.put("1", null);
+	}
+	
 }