rake assets:precompile defaults to production env

actionpack/lib/sprockets/assets.rake

@@ -2,6 +2,7 @@ namespace :assets do
   # Ensures the RAILS_GROUPS environment variable is set
   task :ensure_env do
     ENV["RAILS_GROUPS"] ||= "assets"
+    ENV["RAILS_ENV"]    ||= "production"
   end
 
   desc "Compile all the assets named in config.assets.precompile"