### Before submitting an issue I have first:

- [X] read the documentation on the [homepage](https://github.com/jenkinsci/gitlab-plugin)
- [X] searched for similar already existing issue
- [X] confirmed that my GitLab webhook is using the http://JENKINS_URL/project endpoint and not the http://JENKINS_URL/job endpoint

## Issue

### Context
- **Gitlab plugin version**: 1.5.11
- **Gitlab version**: 11.7.4-ee
- **Jenkins version**: 2.169
- **Job type**: Pipeline

### Logs & Traces
```
com.dabsquared.gitlabjenkins.gitlab.api.impl.ResteasyGitLabClientBuilder$LoggingFilter filter
Call GitLab:
HTTP method: GET
URL: https://gitlab-ee-internal/api/v4/user
Request headers: [
Accept = [application/json],
PRIVATE-TOKEN = [****FILTERED****]
]
com.dabsquared.gitlabjenkins.gitlab.api.impl.ResteasyGitLabClientBuilder$LoggingFilter filter
Got response from GitLab:
URL: https://gitlab-ee-internal/api/v4/user
Status: 400 Bad Request
Response headers: [
Connection = [close],
Content-Length = [246],
Content-Type = [text/html],
Date = [ Wed, 03 Apr 2019 20:36:49 GMT],
Server = [nginx]
]
Response Body:
400 No required SSL certificate was sent

400 Bad Request

No required SSL certificate was sent

---

nginx

```
### Problem description

_Describe your problem in a meaningful way_:
- I'm trying to test the connection in Jenkins->Manage Jenkins->Configure System: GitLab. I've inserted the Connection name, GitLab host URL, and Credentials with the API token. I click the Test Connection button, and the above error happens.
- I'm expecting this to successfully connect to GitLab. Using a curl command to verify this, the Jenkins server certs are able to access https://gitlab-ee-internal/api/v4/user with the Private Token and successfully return back a response. Am I missing a configuration setting to tell Jenkins to use server certs with the GitLab plugin?

Thanks for any help!