Fix JSON deserialization issue (#53)

* Added ClosedBy clas

* Changed ClosedBy type from string to ClosedBy class

src/GitLabApiClient/Models/Issues/Responses/ClosedBy.cs

@@ -0,0 +1,28 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+using Newtonsoft.Json;
+
+namespace GitLabApiClient.Models.Issues.Responses
+{
+    public sealed class ClosedBy : ModifiableObject
+    {
+        [JsonProperty("active")]
+        public string State;
+
+        [JsonProperty("web_url")]
+        public string WebUrl;
+
+        [JsonProperty("avatar_url")]
+        public string AvatarUrl;
+
+        [JsonProperty("username")]
+        public string Username;
+
+        [JsonProperty("id")]
+        public string Id;
+
+        [JsonProperty("name")]
+        public string Name;
+    }
+}

src/GitLabApiClient/Models/Issues/Responses/Issue.cs

@@ -38,7 +38,7 @@ public sealed class Issue : ModifiableObject
         public DateTime? ClosedAt { get; set; }
 
         [JsonProperty("closed_by")]
-        public string ClosedBy { get; set; }
+        public ClosedBy ClosedBy { get; set; }
 
         [JsonProperty("title")]
         public string Title { get; set; }

src/GitLabApiClient/Models/MergeRequests/Responses/MergeRequest.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using GitLabApiClient.Models.Issues.Responses;
 using GitLabApiClient.Models.Milestones.Responses;
 using Newtonsoft.Json;
 
@@ -62,7 +63,7 @@ public sealed class MergeRequest : ModifiableObject
         public DateTime? ClosedAt { get; set; }
 
         [JsonProperty("closed_by")]
-        public string ClosedBy { get; set; }
+        public ClosedBy ClosedBy { get; set; }
 
         [JsonProperty("target_branch")]
         public string TargetBranch { get; set; }