Make ActionMailer::Previews methods class methods.

actionmailer/CHANGELOG.md

+*   Make ActionMailer::Previews methods class methods. Previously they were
+    instance methods and ActionMailer tries to render a message when they
+    are called.
+
+    *Cristian Bica*
+
 *   Deprecate `*_path` helpers in email views. When used they generate
     non-working links and are not the intention of most developers. Instead
     we recommend to use `*_url` helper.

actionmailer/lib/action_mailer/preview.rb

@@ -22,7 +22,9 @@ module Previews #:nodoc:
       # :nodoc:
       mattr_accessor :preview_interceptors, instance_writer: false
       self.preview_interceptors = []
+    end
 
+    module ClassMethods
       # Register one or more Interceptors which will be called before mail is previewed.
       def register_preview_interceptors(*interceptors)
         interceptors.flatten.compact.each { |interceptor| register_preview_interceptor(interceptor) }