Fixed assumption made on session implementations.

In the snippet 'session.setdefault(...).append(...)', it was being
assumed that changes made to mutable structures in the session are
are always in sync with the session object, which is not true for
session implementations that use a external storage for keeping their
keys/values.

flask/helpers.py

@@ -261,7 +261,9 @@ def flash(message, category='message'):
                      messages and ``'warning'`` for warnings.  However any
                      kind of string can be used as category.
     """
-    session.setdefault('_flashes', []).append((category, message))
+    flashes = session.get('_flashes', [])
+    flashes.append((category, message))
+    session['_flashes'] = flashes
 
 
 def get_flashed_messages(with_categories=False, category_filter=[]):