Added missing require, we are using bind method defined on active_support/core_ext/proc

[#4610 state:committed]

Signed-off-by: José Valim <jose.valim@gmail.com>

actionmailer/lib/action_mailer/base.rb

@@ -3,6 +3,7 @@ require 'action_mailer/tmail_compat'
 require 'action_mailer/collector'
 require 'active_support/core_ext/array/wrap'
 require 'active_support/core_ext/object/blank'
+require 'active_support/core_ext/proc'
 
 module ActionMailer #:nodoc:
   # Action Mailer allows you to send email from your application using a mailer model and views.