fix #52 sqlalchemy webdav issue

tracim/lib/webdav/__init__.py

             WsgiDavDirBrowser,
             TracimUserSession,
             HTTPAuthenticator,
-            TracimEnv,
             ErrorPrinter,
             TracimWsgiDavDebugFilter,
+            TracimEnv,
 
         ]
         config['provider_mapping'] = {

tracim/models/__init__.py

 
     """
     dbsession = session_factory()
+    # FIXME - G.M - 02-05-2018 - Check Zope/Sqlalchemy session conf.
+    # We use both keep_session=True for zope and
+    # expire_on_commit=False for sessionmaker to keep session alive after
+    # commit ( in order  to not have trouble like #52 or
+    # detached objects problems).
+    # These problem happened because we use "commit" in our current code.
+    # Understand what those params really mean and check if it can cause
+    # troubles somewhere else.
     zope.sqlalchemy.register(
-        dbsession, transaction_manager=transaction_manager)
+        dbsession,
+        transaction_manager=transaction_manager,
+        keep_session=True,
+    )
     listen(dbsession, 'before_flush', prevent_content_revision_delete)
     return dbsession
 

tracim/tests/library/test_webdav.py

             )
         )
 
-    @pytest.mark.xfail(raises=InvalidRequestError,
-                       reason='Sqlalchemy session and transaction bug')
     def test_unit__create_delete_and_create_file__ok(self):
         provider = self._get_provider(self.app_config)
         environ = self._get_environ(
                 content_to_move.parent.label
         )
 
-    @pytest.mark.xfail(raises=InvalidRequestError,
-                       reason='Sqlalchemy session and transaction bug')
     def test_unit__update_content__ok(self):
         provider = self._get_provider(self.app_config)
         environ = self._get_environ(