Could not retrive pipeline status with [ci-skip] commit message

## Issue

### Context
- **Gitlab plugin version**: 1.5.12
- **Gitlab version**: 12.0.3
- **Jenkins version**: 2.176.1
- **Job type**: Pipeline

### Logs & Traces

```
Jul 14, 2019 12:05:54 PM INFO com.dabsquared.gitlabjenkins.webhook.GitLabWebHook getDynamic
WebHook called with url: /project/myproject
Jul 14, 2019 12:05:55 PM FINE com.dabsquared.gitlabjenkins.webhook.build.MergeRequestBuildAction
MergeRequest: {
"object_kind" : "merge_request",
"event_type" : "merge_request",
"user" : {
"name" : "My User",
"username" : "MyUserName",
"avatar_url" : "https://gitlab.example.com/uploads/-/system/user/avatar/31/avatar.png"
},
"project" : {
"id" : 394,
"name" : "main",
"description" : "",
"web_url" : "https://gitlab.example.com/myproject/backend/main",
"avatar_url" : "https://gitlab.example.com/uploads/-/system/project/avatar/394/1.jpeg",
"git_ssh_url" : "ssh://git@gitlab.example.com:2222/myproject/backend/main.git",
"git_http_url" : "https://gitlab.example.com/myproject/backend/main.git",
"namespace" : "Backend",
"visibility_level" : 0,
"path_with_namespace" : "myproject/backend/main",
"default_branch" : "develop",
"ci_config_path" : null,
"homepage" : "https://gitlab.example.com/myproject/backend/main",
"url" : "ssh://git@gitlab.example.com:2222/myproject/backend/main.git",
"ssh_url" : "ssh://git@gitlab.example.com:2222/myproject/backend/main.git",
"http_url" : "https://gitlab.example.com/myproject/backend/main.git"
},
"object_attributes" : {
"assignee_id" : 31,
"author_id" : 31,
"created_at" : "2019-07-08 17:58:18 UTC",
"description" : "",
"head_pipeline_id" : 5070,
"id" : 5989,
"iid" : 131,
"last_edited_at" : null,
"last_edited_by_id" : null,
"merge_commit_sha" : null,
"merge_error" : null,
"merge_params" : {
"force_remove_source_branch" : "0"
},
"merge_status" : "unchecked",
"merge_user_id" : null,
"merge_when_pipeline_succeeds" : false,
"milestone_id" : null,
"source_branch" : "feature/add_full_automation",
"source_project_id" : 394,
"state" : "opened",
"target_branch" : "develop",
"target_project_id" : 394,
"time_estimate" : 0,
"title" : "[ADD] Full Automation",
"updated_at" : "2019-07-14 09:05:54 UTC",
"updated_by_id" : null,
"url" : "https://gitlab.example.com/myproject/backend/main/merge_requests/131",
"source" : {
"id" : 394,
"name" : "main",
"description" : "",
"web_url" : "https://gitlab.example.com/myproject/backend/main",
"avatar_url" : "https://gitlab.example.com/uploads/-/system/project/avatar/394/1.jpeg",
"git_ssh_url" : "ssh://git@gitlab.example.com:2222/myproject/backend/main.git",
"git_http_url" : "https://gitlab.example.com/myproject/backend/main.git",
"namespace" : "Backend",
"visibility_level" : 0,
"path_with_namespace" : "myproject/backend/main",
"default_branch" : "develop",
"ci_config_path" : null,
"homepage" : "https://gitlab.example.com/myproject/backend/main",
"url" : "ssh://git@gitlab.example.com:2222/myproject/backend/main.git",
"ssh_url" : "ssh://git@gitlab.example.com:2222/myproject/backend/main.git",
"http_url" : "https://gitlab.example.com/myproject/backend/main.git"
},
"target" : {
"id" : 394,
"name" : "main",
"description" : "",
"web_url" : "https://gitlab.example.com/myproject/backend/main",
"avatar_url" : "https://gitlab.example.com/uploads/-/system/project/avatar/394/1.jpeg",
"git_ssh_url" : "ssh://git@gitlab.example.com:2222/myproject/backend/main.git",
"git_http_url" : "https://gitlab.example.com/myproject/backend/main.git",
"namespace" : "Backend",
"visibility_level" : 0,
"path_with_namespace" : "myproject/backend/main",
"default_branch" : "develop",
"ci_config_path" : null,
"homepage" : "https://gitlab.example.com/myproject/backend/main",
"url" : "ssh://git@gitlab.example.com:2222/myproject/backend/main.git",
"ssh_url" : "ssh://git@gitlab.example.com:2222/myproject/backend/main.git",
"http_url" : "https://gitlab.example.com/myproject/backend/main.git"
},
"last_commit" : {
"id" : "1a4e4d9c47436b86f1f71aad1422e24220b6906a",
"message" : "[ADD] Full Automation [ci-skip]\n",
"timestamp" : "2019-07-14T09:03:52Z",
"url" : "https://gitlab.example.com/myproject/backend/main/commit/1a4e4d9c47436b86f1f71aad1422e24220b6906a",
"author" : {
"name" : "My User",
"email" : "MyUserName@gmail.com"
}
},
"work_in_progress" : false,
"total_time_spent" : 0,
"human_total_time_spent" : null,
"human_time_estimate" : null,
"assignee_ids" : [ 31 ],
"action" : "update",
"oldrev" : "3832e47d2f4142898fb58784720f1cb706f9a5c3"
},
"labels" : [ ],
"changes" : {
"total_time_spent" : {
"previous" : null,
"current" : 0
}
},
"repository" : {
"name" : "main",
"url" : "ssh://git@gitlab.example.com:2222/myproject/backend/main.git",
"description" : "",
"homepage" : "https://gitlab.example.com/myproject/backend/main"
},
"assignees" : [ {
"name" : "My User",
"username" : "MyUserName",
"avatar_url" : "https://gitlab.example.com/uploads/-/system/user/avatar/31/avatar.png"
} ]
}
Jul 14, 2019 12:05:55 PM INFO com.dabsquared.gitlabjenkins.trigger.handler.AbstractWebHookTriggerHandler handle
Skipping due to ci-skip.
```

### Problem description

Describe your problem in a meaningful way:
- _What were you doing (simple push, merge request, MR with fork, ...)_
I use MR Events for trigger my builds.
- _What was expected_
I open MR in my project, trigger build on CI with my commit message and this build going success.
Also, in my project i use "Pipelines must succeed" option.
After that, I push new commit in my branch with [ci-skip] message and i've got this message in MR (see picture below).


I expect that MR Status will be untouch (success from previous build status) and that i can click Merge button, but i can't do it.
It's normal behavior for this case?

In addition, starting with GitLab 12 developers changed Assignee functionality and now variable **gitlabMergeRequestAssignee** is coming empty for builds.