add test for #429 (broken with scala 2.13.x)

Author: PJ Fanning

src/test/scala/com/fasterxml/jackson/module/scala/ser/CaseObjectSerializerTest.scala

@@ -0,0 +1,23 @@
+package com.fasterxml.jackson.module.scala.ser
+
+import com.fasterxml.jackson.module.scala.DefaultScalaModule
+import org.junit.runner.RunWith
+import org.scalatestplus.junit.JUnitRunner
+
+case object CaseObjectExample {
+  val field1: String = "test"
+  val field2: Int = 42
+}
+
+@RunWith(classOf[JUnitRunner])
+class CaseObjectSerializerTest extends SerializerTest {
+
+  def module = DefaultScalaModule
+
+  "An ObjectMapper with the DefaultScalaModule" should "serialize a case object as a bean" in {
+    serialize(CaseObjectExample) should (
+       equal ("""{"field1":"test","field2":42}""") or
+       equal ("""{"field2":42, field1":"test"}""")
+    )
+  }
+}