Added event_type property.

Author: gmessner

src/main/java/org/gitlab4j/api/systemhooks/MergeRequestSystemHookEvent.java

@@ -7,6 +7,8 @@ public class MergeRequestSystemHookEvent extends MergeRequestEvent implements Sy
     public static final String X_GITLAB_EVENT = "System Hook";
     public static final String MERGE_REQUEST_EVENT = "merge_request";
 
+    private String eventType;
+
     @Override
     public String getObjectKind() {
         return (MERGE_REQUEST_EVENT);
@@ -16,4 +18,12 @@ public String getObjectKind() {
     public String getEventName() {
         return (MERGE_REQUEST_EVENT);
     }
+
+    public String getEventType() {
+        return eventType;
+    }
+
+    public void setEvent_type(String eventType) {
+        this.eventType = eventType;
+    }
 }

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

@@ -1,5 +1,6 @@
 {
   "object_kind": "merge_request",
+  "event_type": "merge_request",
   "event_name": "merge_request",
   "user": {
     "name": "Administrator",