INT-4049:Fix FileSplitter for Message item

JIRA: https://jira.spring.io/browse/INT-4049

Since the `FileSplitter` can now return `Message<?>` as item for markers, we should ensure the original headers copying, e.g. `file_remoteSession`
Also change `FtpServerOutboundTests-context.xml` to check that new `file_marker` header works well

spring-integration-sftp/src/test/java/org/springframework/integration/sftp/outbound/SftpServerOutboundTests-context.xml

@@ -151,7 +151,7 @@
 	</int:chain>
 
 	<int:service-activator input-channel="markers"
-	  expression="payload.mark.toString().equals('END') ? headers['file_remoteSession'].close() : null"/>
+	  expression="headers['file_marker']?.equals('END') ? headers['file_remoteSession'].close() : null"/>
 
 	<int-sftp:outbound-gateway
 			session-factory="sftpSessionFactory"