Rails core - Sep 2014 - Add support for multiple preview_path

Hi guys.

I'd like to add support for multiple preview_path of ActionMailer.  (It 
could configure only one directory path now.)
Because I think I'd like to configure it in Spree's Rails Engine.
(Spree's
repository https://github.com/spree/spree)

For example.

```
       initializer "spree.core.mailers_previews" do |app|
         app.config.action_mailer.preview_path << File.join(root,
"test",
"mailers", "previews") # => 
spree/core/test/mailers/previews
       end
```

Rails's patch is this.

https://github.com/camelmasa/rails/pull/2/files


What do you think about this ?
I think I'd like to create PR when patch is no problem.

Thanks !

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