Don't use : as an invalid cookie character

Since http://bugs.python.org/issue2193 has been resolved in favour of
the colon in cookie names, we need to test invalid cookie removal using
a different character. "@" is still considered invalid by all sources.

tests/regressiontests/requests/tests.py

@@ -57,7 +57,7 @@ class RequestsTests(unittest.TestCase):
                          str_prefix("<WSGIRequest\npath:/otherpath/,\nGET:{%(_)s'a': %(_)s'b'},\nPOST:{%(_)s'c': %(_)s'd'},\nCOOKIES:{%(_)s'e': %(_)s'f'},\nMETA:{%(_)s'g': %(_)s'h'}>"))
 
     def test_parse_cookie(self):
-        self.assertEqual(parse_cookie('invalid:key=true'), {})
+        self.assertEqual(parse_cookie('invalid@key=true'), {})
 
     def test_httprequest_location(self):
         request = HttpRequest()