[fix] ensure that the prevent default and stop prop occurs in the correct context

reported here https://github.com/marionettejs/backbone.marionette/issues/758#issuecomment-27213784
introduced in a790fb39b9d2bcfb12d6f8f8b6f8d6187e535ff4
Merge pull request #759 from marionettejs/sjs/fix-prevent-default-context-bug

[fix] ensure that the prevent default and stop prop occurs in the correct context