Add :send_core_emails config prefernce

Allow developers to use standard rails mail configuration (in config
files) by setting :override_actionmailer_config to false without sending
spree core emails (e.g. order confirmation). This is useful e.g. in the
case where devs have opted to use an external mail API such as Mandrill
for store-related emails but still want to use ActionMailer in other
parts of their app.

Fixes spree#3812

core/app/mailers/spree/base_mailer.rb

@@ -8,5 +8,10 @@ def money(amount)
       Spree::Money.new(amount).to_s
     end
     helper_method :money
+
+    def mail(headers={}, &block)
+      super if Spree::Config[:send_core_emails]
+    end
+
   end
 end

core/app/models/spree/app_configuration.rb

@@ -85,6 +85,7 @@ class AppConfiguration < Preferences::Configuration
 
     # Default mail headers settings
     preference :enable_mail_delivery, :boolean, :default => false
+    preference :send_core_emails, :boolean, :default => true
     preference :mails_from, :string, :default => '[email protected]'
     preference :mail_bcc, :string, :default => '[email protected]'
     preference :intercept_email, :string, :default => nil

core/spec/mailers/order_mailer_spec.rb

@@ -108,4 +108,13 @@
       end
     end
   end
+
+  context "with preference :send_core_emails set to false" do
+    it "sends no email" do
+      Spree::Config.set(:send_core_emails, false)
+      message = Spree::OrderMailer.confirm_email(order)
+      message.body.should be_blank
+    end
+  end
+
 end