[#819] set RAILS_ENV if only RACK_ENV is provided

config/environment.rb

@@ -18,6 +18,9 @@
 # you don't control web/app server and can't set it the proper way
 # ENV['RAILS_ENV'] ||= 'production'
 
+# use RACK_ENV if we are running as a simple rack app
+ENV['RAILS_ENV'] ||= ENV['RACK_ENV'] if ENV['RACK_ENV']
+
 # Specifies gem version of Rails to use when vendor/rails is not present
 RAILS_GEM_VERSION = '2.3.14' unless defined? RAILS_GEM_VERSION