Zepto#find(somethingFalsey) should return $()

Now, `Zepto#find(somethingFalsey)` return a pure empty array.
But `Zepto#find(somethingFalsey)` should return a `$()`.

Because:
 * We expect `Zepto#find` to return collection
 * jQuery compatibility (for v1.11.1 and v2.1.1)
Introduce `assertZeptoCollection` to verify object is not a plain Array

We often use `assertLength` for checking whether a Zepto collection of
proper size was returned, but in some test a plain Array could also pass
that check. Use `assertZeptoCollection` in places where an object is
only subjected to a `assertLength` check but not any other tests.
Ensure that empty `find()` result is always a Zepto collection
Merge branch 'find-falsey'

References #959