Minor refactoring of SelfAssignment cop

unify offense message
tests should not use the same code its testing
Merge pull request #1082 from geniou/self_assignment_cleanup

Minor refactoring of SelfAssignment cop
Fix Exclude configuration not working on Windows

Check for an absolute in Windows style to avoid prefixing it with the
current directory.
Merge pull request #1076 from wndhydrnt/b-windows-exclude

Fix Exclude configuration not working on Windows