Fix TypeError for class_mapper called w/ iterable

When the class_ passed is not a mapped class but is actually an iterable, the string formatting operation fails with a TypeError, and the expected ArgumentError is not raised. Calling code which is using reflection and expects this error will fail (e.g. the sadisplay module).
Merge pull request #58 from kstark/patch-1

Fix TypeError for class_mapper called w/ iterable