Honor the runner -e toggle even if the RAILS_ENV constant has already been set

git-svn-id: http://svn-commit.rubyonrails.org/rails/trunk@2538 5ecf4fe2-1ee6-0310-87b1-e25e094e27de

railties/lib/commands/runner.rb

@@ -20,7 +20,11 @@ ARGV.options do |opts|
   opts.parse!
 end
 
-ENV["RAILS_ENV"] = options[:environment]
+if defined?(RAILS_ENV)
+  RAILS_ENV.replace(options[:environment])
+else
+  ENV["RAILS_ENV"] = options[:environment]
+end
 
 require RAILS_ROOT + '/config/environment'
-eval(ARGV.first)
+eval(ARGV.first)