Return nil for Mime::NullType#ref

actionpack/lib/action_controller/metal/rendering.rb

@@ -6,7 +6,7 @@ module ActionController
 
     # Before processing, set the request formats in current controller formats.
     def process_action(*) #:nodoc:
-      self.formats = request.formats.select { |x| !x.nil? }.map(&:ref)
+      self.formats = request.formats.map(&:ref).compact
       super
     end
 

actionpack/lib/action_dispatch/http/mime_type.rb

@@ -312,6 +312,10 @@ module Mime
       true
     end
 
+    def ref
+      nil
+    end
+
     def respond_to_missing?(method, include_private = false)
       method.to_s.ends_with? '?'
     end