Add missing toString()

Author: jmini

gitlab4j-models/src/main/java/org/gitlab4j/api/models/Application.java

@@ -2,6 +2,8 @@
 
 import java.io.Serializable;
 
+import org.gitlab4j.models.utils.JacksonJson;
+
 public class Application implements Serializable {
     private static final long serialVersionUID = 1L;
 
@@ -59,4 +61,9 @@ public String getSecret() {
     public void setSecret(String secret) {
         this.secret = secret;
     }
+
+    @Override
+    public String toString() {
+        return (JacksonJson.toJsonString(this));
+    }
 }