Copy hack to keep facet selections when changing relevancy

SearchView just passes request data through to keep facet selections.
It's not a security risk, but still a nasty hack. But as this PR to
improve search has already grown to seach epic proportions, we just
imitate the same thing here.

oscar/apps/search/facets.py

@@ -1,5 +1,4 @@
 from django.conf import settings
-from django.utils.six.moves import map
 from purl import URL
 from haystack.query import SearchQuerySet
 

oscar/apps/search/search_handlers.py

@@ -42,6 +42,7 @@ class SearchHandler(object):
 
     def __init__(self, request_data, full_path):
         self.full_path = full_path
+        self.request_data = request_data
 
         # Triggers the search.
         search_queryset = self.get_search_queryset()
@@ -196,6 +197,11 @@ def get_search_context_data(self, context_object_name=None):
         context = {
             'facet_data': facet_data,
             'has_facets': has_facets,
+            # This is a serious code smell; we just pass through the selected
+            # facets data to the view again, and the template adds those
+            # as fields to the form. This hack ensures that facets stay
+            # selected when changing relevancy.
+            'selected_facets': self.request_data.getlist('selected_facets'),
             'form': self.search_form,
             'paginator': self.paginator,
             'page_obj': self.page,