allow None in requirement

test/test_requires.py

@@ -131,3 +131,20 @@ def test_requires_not_transitive():
     assert props == ['disabled']
     #
     c.ip_address_service_web
+
+
+def test_requires_None():
+    a = BoolOption('activate_service', '')
+    b = IPOption('ip_address_service', '',
+                 requires=[(a, None, 'disabled')])
+    od = OptionDescription('service', '', [a, b])
+    c = Config(od)
+    c.read_write()
+    props = []
+    try:
+        c.ip_address_service
+    except PropertiesOptionError, err:
+        props = err.proptype
+    assert props == ['disabled']
+    c.activate_service = False
+    c.ip_address_service