Fix test error test_assign_range() and test_assign_start_and_length() on ruby-2.x

Array#[range]=nil does not delete element(s) on ruby-1.9 or later.
(see ruby/doc/ChangeLog-1.9.3)

    # ruby-1.8
    >> ary = [1,2,3,4,5]
    => [1, 2, 3, 4, 5]
    >> ary[2..3] = nil
    => nil
    >> ary
    => [1, 2, 5]

    # ruby-2.1
    >> ary = [1,2,3,4,5]
    => [1, 2, 3, 4, 5]
    >> ary[2..3] = nil
    => nil
    >> ary
    => [1, 2, nil, 5]

git-svn-id: https://svn.code.sf.net/p/rubycocoa/svn/trunk/src@2525 f4f36d40-9719-0410-9868-dde021a23eda