Test against the mail gem's edge

The edge version ships with a patch that uses composition over
inheritance for the Mail::PartsList object (see mikel/mail#782).
Let's test Action Mailer against it to prevent eventual regressions
and experience it.

Moreover, this branch makes the Action Mailer suite green against
Rubinius.

Gemfile

@@ -15,6 +15,7 @@ gem 'jquery-rails', github: 'rails/jquery-rails', branch: 'master'
 gem 'coffee-rails', '~> 4.1.0'
 gem 'turbolinks'
 gem 'arel', github: 'rails/arel', branch: 'master'
+gem 'mail', github: 'mikel/mail'
 
 # require: false so bcrypt is loaded only when has_secure_password is used.
 # This is to avoid ActiveModel (and by extension the entire framework)

Gemfile.lock

@@ -11,6 +11,13 @@ GIT
       qu (= 0.2.0)
       redis-namespace
 
+GIT
+  remote: git://github.com/mikel/mail.git
+  revision: b159e0a542962fdd5e292a48cfffa560d7cf412e
+  specs:
+    mail (2.6.3.edge)
+      mime-types (>= 1.16, < 3)
+
 GIT
   remote: git://github.com/rails/arel.git
   revision: aac9da257f291ad8d2d4f914528881c240848bb2
@@ -123,8 +130,6 @@ GEM
       nokogiri
     loofah (2.0.1)
       nokogiri (>= 1.5.9)
-    mail (2.6.3)
-      mime-types (>= 1.16, < 3)
     metaclass (0.0.4)
     mime-types (2.4.3)
     mini_portile (0.6.2)
@@ -250,6 +255,7 @@ DEPENDENCIES
   jquery-rails!
   json
   kindlerb (= 0.1.1)
+  mail!
   minitest (< 5.3.4)
   mocha (~> 0.14)
   mysql (>= 2.9.0)