Chaining composition should not clear the previously defined links

Added a failing test to show desired behavior.

I think this is caused by the `include Roar::Representer::JSON` or `include Roar::Representer::Feature::Hypermedia`, as they might be re-defining the current state instead of reusing it. This is a wild guess; I haven't looked at the code yet.

My use case is re-using existing representers whenever possible. For example:

```ruby
module SomeCollection
  include Roar::Representer::JSON::HAL
  include Pagination
  include Curie
  [...]
end
```

Thoughts?