[BUG][Kotlin-client] Handling default values of parameters (#12255)

* Bugfix Kotlin-client: Handling default values of parameters

* Adding object

samples/client/petstore/kotlin-default-values-multiplatform/README.md

@@ -0,0 +1,41 @@
+# org.openapitools.client - Kotlin client library for Demo
+
+## Requires
+
+* Kotlin 1.5.10
+
+## Build
+
+```
+./gradlew check assemble
+```
+
+This runs all tests and packages the library.
+
+## Features/Implementation Notes
+
+* Supports JSON inputs/outputs, File inputs, and Form inputs.
+* Supports collection formats for query parameters: csv, tsv, ssv, pipes.
+* Some Kotlin and Java types are fully qualified to avoid conflicts with types defined in OpenAPI definitions.
+
+
+<a name="documentation-for-api-endpoints"></a>
+## Documentation for API Endpoints
+
+All URIs are relative to *http://localhost*
+
+Class | Method | HTTP request | Description
+------------ | ------------- | ------------- | -------------
+*DefaultApi* | [**test**](docs/DefaultApi.md#test) | **POST** /test | Tests default values
+
+
+<a name="documentation-for-models"></a>
+## Documentation for Models
+
+ - [org.openapitools.client.models.Apa](docs/Apa.md)
+
+
+<a name="documentation-for-authorization"></a>
+## Documentation for Authorization
+
+All endpoints do not require authorization.