added more unittests for various tasks

--HG--
branch : trunk
diff --git a/tests/test_ext.py b/tests/test_ext.py
index 7da2515..d3364bb 100644
--- a/tests/test_ext.py
+++ b/tests/test_ext.py
@@ -70,3 +70,11 @@
 
 def test_identifier():
     assert TestExtension.identifier == __name__ + '.TestExtension'
+
+
+def test_rebinding():
+    original = Environment(extensions=[TestExtension])
+    overlay = original.overlay()
+    for env in original, overlay:
+        for ext in env.extensions.itervalues():
+            assert ext.environment is env