Solr backend: run() kwargs are passed directly to search backend

This allows customization by subclasses and also fixes #888
by ensuring that the custom field list prepared by 
`ValuesQuerySet` and `ValuesListQuerySet` is actually used.
ElasticSearch backend: run() kwargs are passed directly to search backend

This allows customization by subclasses and also fixes #888
by ensuring that the custom field list prepared by 
`ValuesQuerySet` and `ValuesListQuerySet` is actually used.
Merge pull request #888 from acdha/888-solr-field-list-regression

Solr / ElasticSearch backends: restore run() kwargs handling

This fixes an earlier regression which did not break functionality but made `.values()` and `.values_list()` much less of an optimization than intended.

#925 will be a more comprehensive refactor but this is enough of a performance win to be worth including if a point release happens before #925 lands.