Configuration of webhook acceptance not working

## Issue

### Context
- **Gitlab plugin version**: 1.5.10
- **Gitlab version**: 10.6.2-ee
- **Jenkins version**: 2.107.1
- **Job type**: Pipeline

### Problem description
I am using a Webhook from Gitlab that is triggered by merge request events which in turn triggers a Jenkins job which is configured to only run on opened MRs.
The issue is that this Jenkins job is not only started on opened MRs but also when a new commit is added to an already existing MR.

To add in on this:
The Jenkins job is only started on opened and updated MRs however the settings in the Jenkins job are set. Even if it should only run on closed MRs this is ignored and the job is run on opened and updated MRs.

I read in other issues(#796) that there have been problems with the correct functionality of this particular feature in the past. I don't know how closely / if my problem relates to these old issues.