Fix incorrectly appended square brackets to a multiple select box

If an explicit name has been given and it already ends with "[]"

Before:

    select(:category, [], {}, multiple: true, name: "post[category][]")
    # => <select name="post[category][][]" ...>

After:

    select(:category, [], {}, multiple: true, name: "post[category][]")
    # => <select name="post[category][]" ...>