title: SQLAlchemy 0.6.5 (and 0.6.8) SessionExtension after_flush doesn't gets called
body: <p>I don't get it, but this code doesn't call after_flush/before_flush/after_flush_postexec</p>
  
  <pre><code># -*- coding: utf-8 -*-
  
  from sqlalchemy.orm import scoped_session, sessionmaker
  from sqlalchemy.orm.interfaces import SessionExtension
  
  class AfterFlushExtension(SessionExtension):
      def before_commit(self, session):
          print "&gt; before_commit"
  
      def after_commit(self, session):
          print "&gt; after_commit"
  
      def before_flush(self, session, flush_context, instances):
          print '&gt; before_flush'
  
      def after_flush(self, session, flush_context):
          print '&gt; after_flush'
  
      def after_flush_postexec(self, session, flush_context):
          print '&gt; after_flush_postexec'
  
  session = scoped_session(sessionmaker(extension=AfterFlushExtension()))
  session.flush()
  session.commit()
  </code></pre>
  
  <p>And a result:</p>
  
  <pre><code>$ python ~/Dropbox/playground/python/sqlalchemy_hook_test/main.py 
  &gt; before_commit
  &gt; after_commit
  </code></pre>