Fixes #765 - single => true' on facet now works on "more" screen

Fixes #765
The bug was:
In :solr_facet_params which queries solr when opening the more window, we do this:
```ruby solr_params[:"facet.field"] = [facet_field] ```
The problem is that we override logic that :add_facetting_to_solr is doing for special usecases like query facets, pivot facets and more..

The changes are:
*1 - Seperated some of :add_facetting_to_solr logic to a new method named :handle_advanced_facets
*2 - Now I can call the :handle_advanced_facets method also from :solr_facet_params
*3 - In the overriding line I changed facet_field from single value to an array

The fixes:
*1 - as described in #765.
*2 - this will fix many problems with the 'more' screen we didn't reach.