What I wanted is to reuse :add_facetting_to_solr by sending the field_name with user_params, then in :add_facetting_to_solr return only one field in solr_params[:"facet.field"] if field_name is set.
This way we can delete the override line in :solr_facet_params.

To do that:
*1 - we need to refactor some more code, for example:
```ruby
solr_parameters.append_facet_fields blacklight_config.facet_fields_to_add_to_solr
```
flattens to an array all the facet_fields, we need to change it too.

*2 - I think there is nothing more we need from :add_facetting_to_solr