a timezone of Z means UTC

Adaptation of patch presented here:
http://code.google.com/p/pyiso8601/issues/detail?id=15

colander/iso8601.py

@@ -95,7 +95,7 @@ def parse_timezone(tzstring, default_timezone=UTC):
 
     """
     if tzstring == "Z":
-        return default_timezone
+        return UTC
     # This isn't strictly correct, but it's common to encounter dates without
     # timezones so I'll assume the default (which defaults to UTC).
     # Addresses issue 4.

colander/tests/test_iso8601.py

@@ -71,15 +71,21 @@ class Test_FixedOffset(unittest.TestCase):
         self.assertEqual(result, "<FixedOffset 'oneandahalf'>")
 
 class Test_parse_timezone(unittest.TestCase):
-    def _callFUT(self, tzstring):
+    def _callFUT(self, tzstring, **kw):
         from ..iso8601 import parse_timezone
-        return parse_timezone(tzstring)
+        return parse_timezone(tzstring, **kw)
 
     def test_default_Z(self):
         from ..iso8601 import UTC
         result = self._callFUT('Z')
         self.assertEqual(result, UTC)
 
+    def test_Z_with_default_timezone(self):
+        from ..iso8601 import UTC, FixedOffset
+        tz = FixedOffset(1, 0, 'myname')
+        result = self._callFUT('Z', default_timezone=tz)
+        self.assertEqual(result, UTC)
+
     def test_default_None(self):
         from ..iso8601 import UTC
         result = self._callFUT(None)