`Ember.TargetActionSupport`'s `sendAction` should support `null` as context.
Merge pull request #3353 from sebastianseilund/target-action-support-null-context

`Ember.TargetActionSupport`'s `sendAction` should support `null` as context. This change differentiates between `null` (now considered a valid context) and `undefined`, in which case the `actionContextObject` property or `this` would be used instead.