Refactor #solr_facet_params - Removed override after calling #solr_search_params

Removing the override line in ```#solr_facet_params```, it's very vulnerable to changes.

The changes are:
*1 - ```#facet_fields_to_add_to_solr``` in configuration.rb can now receive as parameter which fields to return
*2 - ```#add_facetting_to_solr``` will now check user_params for :facet_fields and if exists will return these facets only.
*3 - I added
```ruby
next if (key.include?('format_ordered') || key.include?('format_limited'))
```
in one of ```#solr_facet_params``` tests, because it compared ```#solr_search_params``` and ```#solr_facet_params``` which now returns more exact results after my fix: it now does not return format_ordered params because they are not used by solr anyway if there is no facet.field=format_ordered.