Bugfixes

[\#5505](https://github.com/rasahq/rasa/issues/5505): Set default value for `weight_sparsity` in `ResponseSelector` to `0`.

:   This fixes a bug in the default behaviour of `ResponseSelector` which was accidentally introduced in `rasa==1.8.0`.
    Users should update to this version and re-train their models if `ResponseSelector` was used in their pipeline.