Fix incorrectly appended square brackets to a multiple select box

Before:

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

After:

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

Conflicts:
	actionpack/CHANGELOG.md
	actionpack/lib/action_view/helpers/tags/base.rb
	actionpack/test/template/form_options_helper_test.rb