Merge branch 'mm12935' into 'master'

Remove Content-Type override for Mattermost OAuth login

Closes #46810

See merge request gitlab-org/gitlab-ce!28374

changelogs/unreleased/mm12935.yml

+---
+title: Remove Content-Type override for Mattermost OAuth login
+merge_request:
+author: Harrison Healey
+type: removed

lib/mattermost/session.rb

@@ -122,7 +122,7 @@ module Mattermost
 
       @oauth_uri = nil
 
-      response = get('/oauth/gitlab/login', follow_redirects: false, format: 'text/html')
+      response = get('/oauth/gitlab/login', follow_redirects: false)
       return unless (300...400) === response.code
 
       redirect_uri = response.headers['location']