Added test for ValueError in custom assert message with % sign (issue #604)

https://bitbucket.org/hpk42/pytest/issue/604/valueerror-unsupported-format-character-in

testing/test_assertmessage.py

@@ -0,0 +1,18 @@
+
+def test_assert_message_fail():
+    '''
+    Check if custom message with % sign do not raise ValueError
+    Later test can be parametrized with other problematic chars
+    '''
+
+    MESSAGE = 'Message with %'
+       
+    try:
+        assert False, MESSAGE
+    except ValueError, ve:
+        assert False, 'ValueError was raised with the following message: ' \
+            + ve.message
+    except AssertionError, ae:
+        assert MESSAGE == ae.message, 'Assertion message: ' + ae.message \
+            + ' is different than expected: ' + MESSAGE
+