[Kotlin][Client] Encode default values with kotlinx serialization (#19253)

* [Kotlin][Client] Encode default values with kotlinx serialization

* [Kotlin][Client] Encode default values with kotlinx serialization

Author: 4brunu

modules/openapi-generator/src/main/resources/kotlin-client/jvm-common/infrastructure/Serializer.kt.mustache

@@ -143,6 +143,7 @@ import java.util.concurrent.atomic.AtomicLong
     val kotlinxSerializationJson: Json by lazy {
         Json {
             serializersModule = kotlinxSerializationAdapters
+            encodeDefaults = true
             ignoreUnknownKeys = true
             isLenient = true
         }

samples/client/petstore/kotlin-json-request-string/src/main/kotlin/org/openapitools/client/infrastructure/Serializer.kt

@@ -45,6 +45,7 @@ object Serializer {
     val kotlinxSerializationJson: Json by lazy {
         Json {
             serializersModule = kotlinxSerializationAdapters
+            encodeDefaults = true
             ignoreUnknownKeys = true
             isLenient = true
         }

samples/client/petstore/kotlin-retrofit2-kotlinx_serialization/src/main/kotlin/org/openapitools/client/infrastructure/Serializer.kt

@@ -45,6 +45,7 @@ object Serializer {
     val kotlinxSerializationJson: Json by lazy {
         Json {
             serializersModule = kotlinxSerializationAdapters
+            encodeDefaults = true
             ignoreUnknownKeys = true
             isLenient = true
         }

samples/client/petstore/kotlin-uppercase-enum/src/main/kotlin/org/openapitools/client/infrastructure/Serializer.kt

@@ -45,6 +45,7 @@ object Serializer {
     val kotlinxSerializationJson: Json by lazy {
         Json {
             serializersModule = kotlinxSerializationAdapters
+            encodeDefaults = true
             ignoreUnknownKeys = true
             isLenient = true
         }