contains_attribute_value missing __repr__

[ivancrneto]

1. Use mox.contains_attribute_value comparator for a check that would fail
2. Run the test and get error printed

What is the expected output? What do you see instead?
Just like with the other Comparator classes, it should say something that tells
what's wrong. Should contain at least what is checking. Instead it displays a
useless generic Python description of the class instance:

+ function.__call__(<IgnoreArg>, <IgnoreArg>, <mox.ContainsAttributeValue 
instance at 0x8bb4b6c>) -> 0

From that message I cannot tell what attribute/value was expected and is
missing.