better handle varying number of parts for mailer

in the rails2 shim. refs CNVS-9184

test-plan:
 - configure canvas to send outgoing email
 - configure a user with a real eamil address and set notifications for
   this user to asap
 - have a different user initiate a conversation with the first user
 - check for a notification email from the conversation
   - entire email should have content-type multipart/alternative with
     both text and html parts
   - text part should have content-type text/plain
   - html part should have content-type text/html
   - both parts should have charset=utf-8 in their content-type
   - html format should still be displayed by default

Change-Id: I0dd1cf7aad6d51e808438946bce172d256ffceb7
Reviewed-on: https://gerrit.instructure.com/26295
Reviewed-by: Jon Willesen <jonw@instructure.com>
QA-Review: Jon Willesen <jonw@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Jacob Fugal <jacob@instructure.com>

app/models/mailer.rb

@@ -26,6 +26,7 @@ class Mailer < ActionMailer::Base
     class Formatter
       def initialize(target)
         @target = target
+        @parts = []
       end
 
       # e.g. the "render text: ..." in format.html{ render text: ... }
@@ -33,21 +34,30 @@ class Mailer < ActionMailer::Base
         params[:text]
       end
 
-      # e.g. format.html{ render text: ... }
-      def html
-        body = yield self
-        @hasHTML = true
-        @target.content_type 'multipart/alternative'
-        @target.part content_type: 'text/html; charset=utf-8', body: body
+      # e.g. format.text{ render text: ... }
+      def text(&block)
+        @parts << {
+          content_type: 'text/plain; charset=utf-8',
+          body: instance_eval(&block)
+        }
       end
 
-      # e.g. format.text{ render text: ... }
-      def text
-        body = yield self
-        if @hasHTML
-          @target.part content_type: 'text/plain; charset=utf-8', body: body
-        else
-          @target.body body
+      # e.g. format.html{ render text: ... }
+      def html(&block)
+        @parts << {
+          content_type: 'text/html; charset=utf-8',
+          body: instance_eval(&block)
+        }
+      end
+
+      def commit
+        if @parts.size > 1
+          @target.content_type 'multipart/alternative'
+          @parts.each{ |part| @target.part part }
+        elsif @parts.size == 1
+          part = @parts.first
+          @target.content_type part[:content_type]
+          @target.body part[:body]
         end
       end
     end
@@ -61,7 +71,9 @@ class Mailer < ActionMailer::Base
       from params[:from]
       reply_to params[:reply_to]
       subject params[:subject]
-      yield Formatter.new(self)
+      formatter = Formatter.new(self)
+      yield formatter
+      formatter.commit
       self
     end