title: Spring Integration: return a non null reply from an outbound-channel-adapter
body: <p>I have a a channel with 2 outbound-channel-adapters hooked to it, both returning a Message object containing the results of their activities.
  This used to work until an upgrade to SI 2.1 was made. Now, I'm getting an error messages due to the fact that a method is returning a non null value.</p>
  
  <p>Here's a code sample:</p>
  
  <pre><code>&lt;integration:gateway id="notificationGateway"
                       default-request-channel="notificationRecordsFetcher" 
                       default-reply-channel="notifySupplier"
                       service-interface="com.supplier.NotificationGateway"/&gt;
  
  &lt;!-- Submit the file to the supplier--&gt;
  &lt;integration:channel id="notifySupplier"/&gt;
  
  &lt;!-- Handle the secured transfer (first) --&gt;
  &lt;integration:outbound-channel-adapter order="1" channel="notifySupplier" id="SupplierSecuredFileTransfer" ref="SubmitToSupplier" method="submit" /&gt;
  
  &lt;!-- Delete the notification records marked as successful (second) --&gt;
  &lt;integration:outbound-channel-adapter order="2" channel="notifySupplier" id="deleteNotificationRecords" ref="databaseService" method="removeNotificationEntries" /&gt;
  </code></pre>
  
  <p>The adapter identified as "SupplierSecuredFileTransfer" takes a bunch of XML files, each represents a database record and send them using SFTP to a destination folder. It then marks the records that were successfully delivered and send them as a reply to the second adapter identified as "deleteNotificationRecords" which in turn reads the message payload updated by the first one and perform on it.
  Now since the new restriction was introduced, I'm not able to pass information between the adapters.
  Any ideas how to overcome this?</p>