chg: make backend subclassing easier

This change allows the backend build_search_kwargs to 
accept arbitrary extra arguments, making life easier for authors of `SearchQuery` or `SearchBackend` subclasses when they can directly pass a value which is directly supported by the backend search client.

Author: acdha

haystack/backends/__init__.py

@@ -129,7 +129,7 @@ def build_search_kwargs(self, query_string, sort_by=None, start_offset=0, end_of
                             narrow_queries=None, spelling_query=None,
                             within=None, dwithin=None, distance_point=None,
                             models=None, limit_to_registered_models=None,
-                            result_class=None):
+                            result_class=None, **extra_kwargs):
         # A convenience method most backends should include in order to make
         # extension easier.
         raise NotImplementedError

haystack/backends/elasticsearch_backend.py

@@ -257,7 +257,7 @@ def build_search_kwargs(self, query_string, sort_by=None, start_offset=0, end_of
                             narrow_queries=None, spelling_query=None,
                             within=None, dwithin=None, distance_point=None,
                             models=None, limit_to_registered_models=None,
-                            result_class=None):
+                            result_class=None, **extra_kwargs):
         index = haystack.connections[self.connection_alias].get_unified_index()
         content_field = index.document_field
 
@@ -486,6 +486,9 @@ def build_search_kwargs(self, query_string, sort_by=None, start_offset=0, end_of
             else:
                 kwargs['query']['filtered']["filter"] = {"bool": {"must": filters}}
 
+        if extra_kwargs:
+            kwargs.update(extra_kwargs)
+
         return kwargs
 
     @log_query

haystack/backends/solr_backend.py

@@ -147,7 +147,8 @@ def build_search_kwargs(self, query_string, sort_by=None, start_offset=0, end_of
                             narrow_queries=None, spelling_query=None,
                             within=None, dwithin=None, distance_point=None,
                             models=None, limit_to_registered_models=None,
-                            result_class=None, stats=None):
+                            result_class=None, stats=None,
+                            **extra_kwargs):
         kwargs = {'fl': '* score'}
 
         if fields:
@@ -283,6 +284,9 @@ def build_search_kwargs(self, query_string, sort_by=None, start_offset=0, end_of
             # kwargs['fl'] += ' _dist_:geodist()'
             pass
 
+        if extra_kwargs:
+            kwargs.update(extra_kwargs)
+
         return kwargs
 
     def more_like_this(self, model_instance, additional_query_string=None,