API doesn't reflect the use of external issue trackers

We have "Issues" enabled in our GitLab on-premise instance but we use Redmine as an external issue tracker (defined in Project > Services). In the Web interface this setting links all Issue hyperlinks to redmine and we're not able to use the GitLab Issue tracker, as expected.

However, when using this project through the GitLab API we're able to raise issues in the GitLab Issue Tracker. Obviously the API doesn't reflect the activated Services and third-party developers can't disable the GitLab issue tracker in this case.

This came up here: Freshcocoa/Trident#35 /cc @Freshcocoa