Fix incorrectly appended square brackets to a multiple select box

Fix incorrectly appended square brackets to a multiple select box, if an explicit name has been given and it already ends with "[]". Actually, I found this bug when upgrading from 3.2.12 to 3.2.13.rc2, so I'd like this to backport this to 3-2-13.

Formtastic is always providing an explicit name, and it already ends with "[]" for one-to-many associations (see [here](https://github.com/justinfrench/formtastic/blob/master/lib/formtastic/inputs/select_input.rb#L201)).

Before:

```ruby
select(:category, [], {}, {:multiple => true, :name => "post[category][]"})
# => <select name="post[category][][]" ...>
```

After:

```ruby
select(:category, [], {}, {:multiple => true, :name => "post[category][]"})
# => <select name="post[category][]" ...>
```

Probable culprits are 175ba04c on master and 2a6f208e on 3-2-13.

/cc @tenderlove would you like me to open a separate PR with a backport?