don't create empty parts in a multi part email (#267)

app/models/outgoing_message_prototype.rb

@@ -163,12 +163,16 @@ class OutgoingMessagePrototype
       if @html_body.blank? && attachments.empty?
         mail.body = @plain_body
       else
+        if !@plain_body.blank?
           mail.text_part = Mail::Part.new
           mail.text_part.body = @plain_body
+        end
+        if !@html_body.blank?
           mail.html_part = Mail::Part.new
           mail.html_part.content_type = "text/html; charset=UTF-8"
           mail.html_part.body = @html_body
         end
+      end
       attachments.each do |attachment|
         mail.attachments[attachment[:name]] = {
           :mime_type => attachment[:content_type],