Added MergeRequest.mergeError (#444) (#445)

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

@@ -31,6 +31,7 @@ public class MergeRequest {
     private Date mergedAt;
     private Participant mergedBy;
     private Boolean mergeWhenPipelineSucceeds;
+    private String mergeError;
     private Milestone milestone;
     private Integer projectId;
     private String sha;
@@ -228,6 +229,14 @@ public class MergeRequest {
         this.mergeWhenPipelineSucceeds = mergeWhenPipelineSucceeds;
     }
 
+    public String getMergeError() {
+        return mergeError;
+    }
+
+    public void setMergeError(String mergeError) {
+        this.mergeError = mergeError;
+    }
+
     public Milestone getMilestone() {
         return milestone;
     }

src/test/resources/org/gitlab4j/api/merge-request.json

@@ -19,6 +19,7 @@
         "avatar_url": "https://www.google.com/",
         "web_url": "https://www.google.com/"
     },
+    "merge_error": "Merge request is not mergeable",
     "author":{
         "id":1,
         "username": "admin",