Attributes not available in didRender

[simonihmig]

I ran into a strange issue, when refactoring ember-popper to support splattributes. Although the polyfill was added, tests fail for Ember 2.18, but passes for newer versions not requiring the polyfill (see kybishop/ember-popper#101, https://travis-ci.org/kybishop/ember-popper/builds/521966875).

I tracked this down to the attributes not being applied to the element at the time didRender is called (which causes other JS errors down the road as the component cannot find the DOM element by ID).

Here's a failing test: #65.
Fails for Ember < 3.4, passes for the newer ones with native support.