[BUGFIX beta] Allow setting of `undefined` value to a `content` property

Fixes setting of `undefined` value to a content property. 

Still, checking `obj[keyName] === value` in these [lines] (https://github.com/selvagsz/ember.js/blob/master/packages_es6/ember-metal/lib/property_set.js#L52-54) is bypassed in two cases

1. If the setter value is `undefined`
2. If the setter property lies inside the proxied content

Using `get(obj, keyName)` might resolve. But was quite nervous to do that as it may involve some additional function calls

Fixes #4644