[Ruby 1.9] Fix broken ip_range unit tests

`assert` is incorrectly used here, since it just tests that the first
argument is `true`, which in Ruby means _any_ object (eg. 0, 1, "", [],
{}) except `false` and `nil`.

Ruby 1.9 is stricter here and checks that the second argument is a
`String` or `Proc`, thus uncovering the bug/typo. `assert_equal` should
be used here instead.