Fixed ActionView::FormOptionsHelper#select with :multiple => false

[bogdan]

The following code:

select :post, :category_ids, [], {}, :multiple => false

Generates the select with name post[category_id][]. This is kinda confusing to add [] in case of false and nil for multiple attribute.

In real example with conditional multiple key we need the following:

select ..., {:class => "gg").merge(my_condition ? {:multiple => true}, {})

That is not very natural. So fixed default name to add [] only when :multiple looks truly.