title: MessageGroupStoreReaper fails with "was expecting a single payload" IllegalStateException
body: <p>I have a simple Spring Integration 2.0.1 aggregator setup, using a SimpleMessageStore along with a regular Spring MessageGroupStoreReaper defined, in order to be able to implement a grouping timeout mechanism (which worked simply as an aggregator 'timeout' parameter in Spring Integration 1.0.4).</p>
  
  <p>I already debugged and verified the messages are aggregated correctly within SimpleMessageStore based on groupId, but upon timeout the MessageGroupStoreReaper fails, with "Unable to access property 'messages' through getter" AccessException. The precise error is located within <a href="http://git.springsource.org/spring-integration/spring-integration/blobs/ff7c0fe3096d3dc36c890cd7abea6e8060b0c5c8/spring-integration-core/src/main/java/org/springframework/integration/util/MessagingMethodInvokerHelper.java" rel="nofollow">MessagingMethodInvokerHelper class</a>, which asserts the messages field is not null. It seems somehow the messages aggregated are not available during execution of the Invoker, resulting in the ""Invalid method parameter for messages: was expecting a single payload." IllegalStateException.</p>
  
  <p>What could be the cause of this issue and how to resolve it?</p>
  
  <p>I already tried updating to 2.0.6, but the issue remains.</p>
  
  <p>***EDIT
  I updated my SI dependencies to 2.2.5, but this did not resolve my problem. I use Spring 3.0.7.</p>