Initialize Templates::ContentTyped @preferred_extension in to nil to avoid ruby warning about it not being initialized.

lib/sinatra/base.rb

@@ -658,6 +658,7 @@ module Sinatra
     def initialize
       super
       @default_layout = :layout
+      @preferred_extension = nil
     end
 
     def erb(template, options = {}, locals = {}, &block)