Always call 'to_liquid' on staff in map filter and allow to_liquid to be called on drops

lib/liquid/drop.rb

@@ -54,7 +54,7 @@ module Liquid
 
     # Check for method existence without invoking respond_to?, which creates symbols
     def self.invokable?(method_name)
-      @invokable_methods ||= Set.new((public_instance_methods - Liquid::Drop.public_instance_methods).map(&:to_s))
+      @invokable_methods ||= Set.new(["to_liquid"] + (public_instance_methods - Liquid::Drop.public_instance_methods).map(&:to_s))
       @invokable_methods.include?(method_name.to_s)
     end
   end

lib/liquid/standardfilters.rb

@@ -101,7 +101,13 @@ module Liquid
     def map(input, property)
       ary = [input].flatten
       ary.map do |e|
-        e.respond_to?('[]') ? e[property] : nil
+        if e.respond_to?(:to_liquid)
+          e = e.to_liquid
+        end
+
+        if e.respond_to?('[]')
+          e[property]
+        end
       end
     end