Index: builders.rb
===================================================================
--- builders.rb	(revision 15949)
+++ builders.rb	(working copy)
@@ -22,6 +22,7 @@
   module Views
     module Builders
       def self.for(format, request, response, &block)
+        format ||= request.format.symbol.to_s unless request.nil?
         builder = case format
           when 'xml',  :xml;  Builders::Xml.new(request, response)
           when 'json', :json; Builders::Json.new(request, response)