add simplified version of Marshallable trait

Author: SergeiIonin

core-scala/src/main/scala/com/baeldung/scala/classcompositionwithtraits/CarTraits.scala

@@ -22,6 +22,14 @@ object CarTraits {
   /**
    * Another trait to be mixed into the class
    */
+  trait SimpleMarshaller extends Car {
+    def toJson: String = s"{${"\"model\""}:$model," +
+      s"\n${"\"horsePower\""}:$horsePower}"
+  }
+
+  /**
+   * Another trait with self-type to be mixed into the class
+   */
   trait Marshaller {
     self: Car =>
     def toJson = s"{${"\"model\""}:$model," +

core-scala/src/test/scala/com/baeldung/scala/classcompositionwithtraits/TraitsTest.scala

@@ -28,6 +28,14 @@ class TraitsTest extends WordSpec with Matchers {
     }
   }
 
+  "Classes that extends Car with SimpleMarshaller" should {
+    "inherit abstract class fields and methods" +
+      "and be marshallable" in {
+      val bmw0 = new BMW0("F15", 309) with SimpleMarshaller
+      bmw0.toJson shouldBe "{\"model\":F15,\n\"horsePower\":309}"
+    }
+  }
+
   "Classes that extends Car with Marshaller" should {
     "inherit abstract class fields and methods" +
       "and be marshallable" in {