Update GitLab MergeRequest's Labels to be an array vs a single value

NuKeeper.Gitlab/GitlabPlatform.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using System.Linq;
 using System.Net.Http;
 using System.Threading.Tasks;
 using NuKeeper.Abstractions.CollaborationModels;
@@ -51,7 +52,7 @@ public async Task OpenPullRequest(ForkData target, PullRequestRequest request, I
                 TargetBranch = request.BaseRef,
                 Id = $"{projectName}/{repositoryName}",
                 RemoveSourceBranch = request.DeleteBranchAfterMerge,
-                Labels = labels.JoinWithCommas()
+                Labels = labels.ToList()
             };
 
             await _client.OpenMergeRequest(projectName, repositoryName, mergeRequest);

NuKeeper.Gitlab/Model/MergeRequest.cs

@@ -1,3 +1,4 @@
+using System.Collections.Generic;
 using Newtonsoft.Json;
 
 namespace NuKeeper.Gitlab.Model
@@ -23,6 +24,6 @@ public class MergeRequest
         public bool RemoveSourceBranch { get; set; }
 
         [JsonProperty("labels")]
-        public string Labels { get; set; }
+        public IList<string> Labels { get; set; }
     }
 }