Fix Collection::implode with \Stringable objects

[timkelty]

Was incorrectly checking for \Illuminate\Support\Stringable.

[sebdesign]

@taylorotwell This PR breaks implode on eloquent collections!

The Model implements the \Stringable interface, so the condition is never truthy when the collection contains eloquent models.

E.g. calling something like $posts->implode('title', ',') no longer implodes the plucked titles from the post models.