Fix insert enumerable

[baarkerlounger]

What if value assigned is Enumerable, but is not Array or Range? (Say, it is ActiveRecord::Relation.) What user expects is it would be converted to_a, and then inserted as a vector. What actually will happen, is pretty obscure error:

when Array, Range would be skipped;
in prepare_value_for_insert, there would be called Array(relation), which will do to_a (producing array of objects)... and then multiplied by DF size, producing vector much larger than expected.

[zverok]

Let's add a small test for the new behavior? With, say, standard library's Set class.

[zverok]

Thanks! 👍