Added created_at property (#594)

src/main/java/org/gitlab4j/api/models/Group.java

@@ -66,6 +66,7 @@ public class Group {
     private Statistics statistics;
     private List<Project> projects;
     private List<Project> sharedProjects;
+    private Date createdAt;
 
     @JsonSerialize(using = JacksonJson.DateOnlySerializer.class)
     private Date markedForDeletionOn;
@@ -206,6 +207,14 @@ public class Group {
         this.markedForDeletionOn = markedForDeletionOn;
     }
 
+    public Date getCreatedAt() {
+	return createdAt;
+    }
+
+    public void setCreatedAt(Date createdAt) {
+	this.createdAt = createdAt;
+    }
+
     public Group withId(Integer id) {
         this.id = id;
         return this;

src/test/resources/org/gitlab4j/api/group.json

@@ -52,7 +52,8 @@
       "open_issues_count": 3,
       "public_jobs": true,
       "shared_with_groups": [],
-      "request_access_enabled": false
+      "request_access_enabled": false,
+      "created_at": "2020-01-15T12:36:29.590Z"
     },
     {
       "id": 6,