Include Draper::ViewContextFilter in ActionMailer::Base

* Only send the :before_filter method if it exists
* This will allow decorators to be used when rendering email

Readme.markdown

@@ -229,6 +229,21 @@ Use the new methods in your views like any other model method (ex: `@article.pub
 <h1><%= @article.title %> <%= @article.published_at %></h1>
 ```
 
+### Using in Mailers
+
+To use decorators in mailers that use helpers, you have to call `set_current_view_context` in your
+mailer method:
+
+```ruby
+class ActicleMailer < ActionMailer::Base
+  def new_article(article)
+    set_current_view_context
+    @article_decorator = ArticleDecorator.decorate(article)
+    mail(:to => 'come@me.bro', :subject => "New Article: #{@article_decorator.title}")
+  end
+end
+```
+
 ## Possible Decoration Methods
 
 Here are some ideas of what you might do in decorator methods:

lib/draper/system.rb

@@ -1,7 +1,8 @@
 module Draper
-  class System    
+  class System
     def self.setup
       ActionController::Base.send(:include, Draper::ViewContextFilter) if defined?(ActionController::Base)
+      ActionMailer::Base.send(:include, Draper::ViewContextFilter) if defined?(ActionMailer::Base)
     end
   end
-end
+end

lib/draper/view_context.rb

@@ -8,14 +8,14 @@ module Draper
       Thread.current[:current_view_context] = input
     end
   end
-  
+
   module ViewContextFilter
     def set_current_view_context
       Draper::ViewContext.current = self.view_context
     end
-    
+
     def self.included(source)
-      source.send(:before_filter, :set_current_view_context)
+      source.send(:before_filter, :set_current_view_context) if source.respond_to?(:before_filter)
     end
   end
-end
+end