feat: 增加trans_value，支持特殊值

lib/ransack_mongo/query.rb

@@ -20,7 +20,7 @@ module RansackMongo
                 or_query(db_adapter, attr, value, p)
               else
                 # attr => name
-                db_adapter.send("#{p}_matcher", attr, value)
+                db_adapter.send("#{p}_matcher", attr, trans_value(value))
               end
             rescue NoMethodError => e
               raise MatcherNotFound, "The matcher #{p} `#{p}_matcher` was not found in the #{db_adapter_class.name}. Check `#{db_adapter_class}.predicates`"
@@ -44,10 +44,23 @@ module RansackMongo
       def or_query(db_adapter, attr, value, p)
         db_adapter.or_op do
           attr.split('_or_').each do |or_attr|
-            db_adapter.send("#{p}_matcher", or_attr, value)
+            db_adapter.send("#{p}_matcher", or_attr, trans_value(value))
           end
         end
       end
+
+      def trans_value value
+        case value
+        when 'true'
+          true
+        when 'false'
+          false
+        when 'null', 'nil'
+          nil
+        else
+          value
+        end
+      end
     end
   end
 end