Merge branch 'ce-3092-fix-unassigned-unapproved-mr-email-failing' into 'master'

Fix unapproved unassigned merge request e-mails failing -- CE backport

Closes gitlab-ee#3092

See merge request gitlab-org/gitlab-ce!18007

spec/mailers/previews/notify_preview.rb

@@ -65,7 +65,7 @@ class NotifyPreview < ActionMailer::Preview
   end
 
   def merge_request
-    @merge_request ||= project.merge_requests.find_by(source_branch: 'master', target_branch: 'feature')
+    @merge_request ||= project.merge_requests.first
   end
 
   def user