emberjs/ember.js — pull request #9908 by mixonic (2014-12-13 20:03:04+00:00)
Title: [BUGFIX beta] Do not stringify class values before they are passed to concat

Fixes #9893.

falsy values from class bindings were beings made into strings before going through the concat helper.

@mmun here I've added `joinWith` as a hash param to `concat`. As an alternative I could spin out the relevant parts of concat out into a utils method and not change the API of the hook.