[BUG][kotlin] Make Kotlin client generate comma separated list for array path params

Created by: sjoblomj

resolve #7199 (closed)

The Kotlin client codegen currently cannot handle path params of type array. The code being generated for the path is currently this: path = "/stuff/{ids}".replace("{"+"ids"+"}", "$ids"). This works fine, but not if ids is a list or an array, since the toString() method is used to create the string.

With the current code, if ids contains 123 and 456, then path would be set to:

• /stuff/[123,456] if collectionType is list.
• /stuff/[Ljava.lang.String;@6a98f353 if collectionType is array.

In both cases, /stuff/123,456 is the expected result.

If we change the code from path = "/stuff/{ids}".replace("{"+"ids"+"}", "$ids") to path = "/stuff/{ids}".replace("{"+"ids"+"}", ids.joinToString(",")), we get the expected result.

OpenAPI Spec demonstrating this:

openapi: 3.0.0
info:
  version: 1.0.0
  title: Demo
paths:
  '/{ids}':
    get:
      parameters:
        - name: ids
          in: path
          required: true
          schema:
            type: array
            items:
              type: string
          style: simple
      responses:
        200:
          description: Successful operation

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