Raise ArgumentError by `NSArray#[range]=nil` on ruby-2.x

    # ruby-1.8 deletes elements when the given value is nil
    ary = [1, 2, 3, 4, 5]
    ary[1..2] = nil
    ary # => [1, 4, 5]

    # ruby-2.1 sets nil
    ary = [1, 2, 3, 4, 5]
    ary[1..2] = nil
    ary # => [1, nil, 4, 5]
    # NSArray cannot contain nil value.
    nsary = OSX::NSArray.arrayWithArray([1, 2, 3, 4, 5])
    nsary[1..2] = nil # => ArgumentError

git-svn-id: https://svn.code.sf.net/p/rubycocoa/svn/trunk/src@2556 f4f36d40-9719-0410-9868-dde021a23eda