Map ancestors to strings before checking for ForbiddenAttributesProtection class

Following-up on PR #16, if strong_parameters isn't installed, this would raise `uninitialized constant ActiveModel::ForbiddenAttributesProtection`.

It may looks hack-ish to map ancestors to strings, but solves the problem, otherwise if/unless condition statements would look a bit ugly and redundant. I could've refactored it into couple of methods but I didn't want to pollute the model classes.