fix scala-akka java8 serializers

Author: chameleon82

modules/openapi-generator/src/main/resources/scala-akka-client/serializers.mustache

@@ -28,7 +28,8 @@ object Serializers {
   case object LocalDateSerializer extends CustomSerializer[LocalDate]( _ => ( {
     case JString(s) => LocalDate.parse(s)
   }, {
-    JString(d.format(DateTimeFormatter.ISO_LOCAL_DATE))
+    case d: LocalDate =>
+      JString(d.format(DateTimeFormatter.ISO_LOCAL_DATE))
   }))
 {{/java8}}
 {{#joda}}