Make issue due email more consistent with other mailers

app/views/notify/issue_due_email.html.haml

-- if Gitlab::CurrentSettings.email_author_in_body
-  %p.details
+%p.details
   #{link_to @issue.author_name, user_url(@issue.author)}'s issue is due soon.
 
 - if @issue.assignees.any?
   %p
     Assignee: #{@issue.assignee_list}
-
 %p
-  This issue is due on: #{@issue.due_date}
+  This issue is due on: #{@issue.due_date.to_s(:medium)}
 
 - if @issue.description
   %div

spec/services/notification_service_spec.rb

@@ -936,6 +936,8 @@ describe NotificationService, :mailer do
 
     describe '#issue_due' do
       before do
+        issue.update!(due_date: Date.today)
+
         update_custom_notification(:issue_due, @u_guest_custom, resource: project)
         update_custom_notification(:issue_due, @u_custom_global)
       end