Added toString() implementation.

src/main/java/org/gitlab4j/api/services/NotificationService.java

@@ -5,6 +5,7 @@ import java.util.HashMap;
 import java.util.Map;
 
 import org.gitlab4j.api.GitLabApiForm;
+import org.gitlab4j.api.utils.JacksonJson;
 
 import com.fasterxml.jackson.annotation.JsonIgnore;
 
@@ -279,4 +280,9 @@ public abstract class NotificationService {
 
         properties.put(prop, value);
     }
+
+    @Override
+    public String toString() {
+        return (JacksonJson.toJsonString(this));
+    }
 }