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

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

@@ -64,7 +64,9 @@ public class ExecutionContext {
 	}
 
 	public void put(String key, Object value) {
-		Assert.isInstanceOf(Serializable.class, value, "Value: [ " + value + "must be serializable.");
+		if(value != null){
+			Assert.isInstanceOf(Serializable.class, value, "Value: [ " + value + "must be serializable.");
+		}
 		dirty = true;
 		map.put(key, value);
 	}

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);
+	}
+	
 }