Check attributesForFaceting with searchable

[Jerska]

A user is reporting issues using attributesForFaceting with the new searchable modifier for facets, while setting it in the dashboard worked fine.
I'm unsure why it didn't work, but setting up a reproduction rails app should be straightforward.

[redox]

Closing it until reproduction case because I cannot see why it would fail.

[ersinakinci]

I had this exact same problem. Turns out that the client wasn't updating my facets due to a misconfigured setting elsewhere that was silently failing (see #383).