Add support for multiple assignees in Merge Requests (#504)

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

@@ -14,6 +14,7 @@ public class MergeRequest {
     private Boolean allowMaintainerToPush;
     private Integer approvalsBeforeMerge;
     private Assignee assignee;
+    private List<Assignee> assignees;
     private Author author;
     private List<Diff> changes;
     private Date closedAt;
@@ -94,6 +95,14 @@ public class MergeRequest {
         this.assignee = assignee;
     }
 
+    public List<Assignee> getAssignees() {
+        return assignees;
+    }
+
+    public void setAssignees(List<Assignee> assignees) {
+        this.assignees = assignees;
+    }
+
     public Author getAuthor() {
         return author;
     }