Fixed to pass with edge version of Underscore

Due to changes in https://github.com/documentcloud/underscore/commit/0ab5bdccd41e55fb02331ec7d2d0053e0bdac9e3 need to fix Backbone.Model test. It would be helpful because some people may use Backbone's unit tests in their Backbone-related projects (plugins for example).
Update due to recomendations
Merge pull request #2282 from DreamTheater/patch-1

Fixed to pass with edge version of Underscore