fixes #781
Merge pull request #782 from amitree/master

Stubbing a method on any_instance_of(SomeModule) gives spurious error about prepended modules
changelog for #781
[skip ci]
Use is_a_matcher? in arg_has_valid_description

This fixes #685. It means that we only print descriptions in errors for
matcher objects and not for user objects.
Implement diffing between args for mock expectations.