Add equal value spec for unrelated objects

We fixed a similar issue on equalizer already. IHMO we should use
equalizer also on veritas and this spec is to highlight the problem.

https://github.com/dkubb/equalizer/pull/7

I do not expect this is gets merged, only opening to start the
discussion.

```ruby
header = Veritas::Relation::Header.coerce([[:foo, String]])
relation = Veritas::Relation.new(header, [])
relation == Object.new # blows up and IMHO should return false
```