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

This allows customization by subclasses and also fixes django-haystack#888
by ensuring that the custom field list prepared by 
`ValuesQuerySet` and `ValuesListQuerySet` is actually used.

Author: acdha

haystack/backends/solr_backend.py

@@ -690,6 +690,9 @@ def run(self, spelling_query=None, **kwargs):
         final_query = self.build_query()
         search_kwargs = self.build_params(spelling_query, **kwargs)
 
+        if kwargs:
+            search_kwargs.update(kwargs)
+
         results = self.backend.search(final_query, **search_kwargs)
         self._results = results.get('results', [])
         self._hit_count = results.get('hits', 0)