Merge all route asset configurations for which regular expression matches

This PR is inspired by #68. But that PR had no test; contained errors and the user was not active. Thus I decided to take matters in own hand.

What does it do? It merges the asset configurations for which the expression matches the current route. This avoids a lot of repetition when definining assets.

What was wrong in the previous PR? The `$default` was not returned when no asset collection got matched.