Concatenate association to be preloaded

To be consistent with how we add associations to be preloaded, we
concatenate instead of pushing.

This also avoids accidental side effects of mutation.

ee/app/controllers/concerns/ee/issuable_collections.rb

@@ -11,7 +11,7 @@ module EE
                                   when 'MergeRequest'
                                     super + [approval_rules: [:users, :group_users], approval_project_rules: [:users, :group_users]]
                                   when 'Issue'
-                                    super.push(*issue_preloads)
+                                    super + issue_preloads
                                   else
                                     super
                                   end