Milestone merge_requests api endpoint returns 404

Summary

Merge requests for a milestone cannot be retrieved via API endpoint (/projects/:id/milestones/:milestone_id/merge_requests)

Steps to reproduce

1. Create a milestone
2. Add a merge request to this milestone
3. Call API endpoint /projects/:id/milestones/:milestone_id/merge_requests

What is the current bug behavior?

Returns 404

What is the expected correct behavior?

Return the merge_ request or an empty array in case the milestone has no merge requests

Output of checks

This bug happens on GitLab.com

Results of GitLab environment info

System information
System:		Ubuntu 16.04
Current User:	git
Using RVM:	no
Ruby Version:	2.3.3p222
Gem Version:	2.6.6
Bundler Version:1.13.7
Rake Version:	10.5.0
Redis Version:	3.2.5
Sidekiq Version:4.2.7

GitLab information
Version:	8.17.3
Revision:	f0c970e
Directory:	/opt/gitlab/embedded/service/gitlab-rails
DB Adapter:	postgresql
URL:		http://gigikasDev
HTTP Clone URL:	http://gigikasDev/some-group/some-project.git
SSH Clone URL:	git@gigikasDev:some-group/some-project.git
Using LDAP:	no
Using Omniauth:	no

GitLab Shell
Version:	4.1.1
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

Checking GitLab Shell ...

GitLab Shell version >= 4.1.1 ? ... OK (4.1.1)
Repo base directory exists?
default... yes
Repo storage directories are symlinks?
default... no
Repo paths owned by git:git?
default... yes
Repo paths access is drwxrws---?
default... yes
hooks directories in repos are links: ... 
1/1 ... ok
Running /opt/gitlab/embedded/service/gitlab-shell/bin/check
Check GitLab API access: OK
Access to /var/opt/gitlab/.ssh/authorized_keys: OK
Send ping to redis server: OK
gitlab-shell self-check successful

Checking GitLab Shell ... Finished

Checking Sidekiq ...

Running? ... yes
Number of Sidekiq processes ... 1

Checking Sidekiq ... Finished

Checking Reply by email ...

Reply by email is disabled in config/gitlab.yml

Checking Reply by email ... Finished

Checking LDAP ...

LDAP is disabled in config/gitlab.yml

Checking LDAP ... Finished

Checking GitLab ...

Git configured with autocrlf=input? ... yes
Database config exists? ... yes
All migrations up? ... yes
Database contains orphaned GroupMembers? ... no
GitLab config exists? ... yes
GitLab config outdated? ... no
Log directory writable? ... yes
Tmp directory writable? ... yes
Uploads directory setup correctly? ... skipped (no tmp uploads folder yet)
Init script exists? ... skipped (omnibus-gitlab has no init script)
Init script up-to-date? ... skipped (omnibus-gitlab has no init script)
projects have namespace: ... 
1/1 ... yes
Redis version >= 2.8.0? ... yes
Ruby version >= 2.1.0 ? ... yes (2.3.3)
Your git bin path is "/opt/gitlab/embedded/bin/git"
Git version >= 2.7.3 ? ... yes (2.10.2)
Active users: 1

Checking GitLab ... Finished