speed up scoped_loaded_collection in item search a bit

Reduces items#index.json by about 10ms per request on my box, development with
cache_classes=true. The time isn't huge, but is worthwhile.

app/flex/flex_search_extender.rb

@@ -11,22 +11,23 @@ module FlexSearchExtender
   def scoped_loaded_collection(options)
     options[:scopes] ||= {}
     @loaded_collection ||= begin
-      records  = []
+      records_by_class_and_id_str = {}
       # returns a structure like {Comment=>[{"_id"=>"123", ...}, {...}], BlogPost=>[...]}
-      h = Flex::Utils.group_array_by(collection) do |d|
-        d.mapped_class(should_raise=true)
-      end
+      h = collection.group_by { |d| d.mapped_class(should_raise=true) }
       h.each do |klass, docs|
+        record_ids = docs.map(&:_id)
         scope = options[:scopes][klass.name] || klass.scoped
-        records |= scope.find(docs.map(&:_id))
-      end
-      class_ids = collection.map { |d| [d.mapped_class.to_s,  d._id] }
-        # Reorder records to preserve order from search results
-        records = class_ids.map do |class_str, id|
-          records.detect do |record|
-          record.class.to_s == class_str && record.id.to_s == id.to_s
+        records = scope.find(record_ids)
+        records.each do |record|
+          records_by_class_and_id_str[record.class] ||= {}
+          records_by_class_and_id_str[record.class][record.id.to_s] = record
         end
       end
+
+      # Reorder records to preserve order from search results
+      records = collection.map do |d|
+        records_by_class_and_id_str[d.mapped_class][d._id]
+      end
       records.extend Flex::Result::Collection
       records.setup(self['hits']['total'], variables)
       records