Pipeline Triggers -> Webhook not working for Private Projects without login!

Summary

Triggering a Pipeline through webhook (e.g. https://gitlab.example.com/api/v4/projects/2/ref/master/trigger/pipeline?token=6490380d24da4ce4da76b58abab022) does only work if the user is authenticated for private projects.

Steps to reproduce

Create a Pipeline Trigger, try to run it through a WebHook trigger.

What is the current bug behavior?

The URL return a 404 as JSON:

{"error":"404 Not Found"}

What is the expected correct behavior?

To work, as it currently works with other trigger methods like POST.

Results of GitLab environment info

Clean install of latest GitLab Version.

Expand for output related to GitLab environment info

System information
System:         Ubuntu 16.04
Proxy:          no
Current User:   git
Using RVM:      no
Ruby Version:   2.3.5p376
Gem Version:    2.6.13
Bundler Version:1.13.7
Rake Version:   12.1.0
Redis Version:  3.2.5
Git Version:    2.13.6
Sidekiq Version:5.0.4
Go Version:     unknown
GitLab information
Version:        10.1.4-ee
Revision:       a512054
Directory:      /opt/gitlab/embedded/service/gitlab-rails
DB Adapter:     postgresql
DB Version:     9.6.5
Elasticsearch:  no
Geo:            no
Using LDAP:     no
Using Omniauth: yes
Omniauth Providers: github
GitLab Shell
Version:        5.9.3
Repository storage paths:

• default: /var/opt/gitlab/git-data/repositories Hooks: /opt/gitlab/embedded/service/gitlab-shell/hooks Git: /opt/gitlab/embedded/bin/git

Results of GitLab application Check

Expand for output related to the GitLab application check

all green!