[Kotlin] fix #20231, OkHttp client can handle a field with a list of files

[jorgeferdz]

issue-20231: Kotlin OkHttp client does not process correctly when a field in the endpoint is defined as an array of lists like this:

...
requestBody:
  content:
    multipart/form-data:
      schema:
        type: object
        properties:
          attachments:
            type: array
            description: attached files
            items:
              type: string
              format: binary
...

If the autogenerated client is used, the server will receive always 1 item in the list but it is not a file or anything and it is trying to serialize like a normal object.

I noticed this issue while developing my own project, so I modified this code (by looking at the Java counterpart that can be seen here) and tested it by building the project in my local machine and adding the plugin to my project manually.

I tried to add some testing (I added the faulty specification) but I was not really sure how to do it. Also by reading the documentation, it didn't find it very clearly what I should do. Any guidance regarding how to add test for that would be very much appreciated.

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[wing328]

suggestion: what about adding docstrings to these new functions?

[jorgeferdz]

Sure! I'll add them in a bit :-)

[wing328]

how is this new test spec used in the PR?

did you intend to create a config file under ./bin/config to test with this spec?

since the change only affects ApiClient.kt so I don't think a new sample is needed to cover this case

we can use the echo server to test it to confirm the request looks good

please ping me via Slack if you want to go with this approach

https://join.slack.com/t/openapi-generator/shared_invite/zt-2uoef5v0g-XGwo8~2oJ3EoziDSO1CmdQ

[jorgeferdz]

Sure! I will ping you. As I mentioned, I didn't really get a clear understanding on how to add the tests to check this fix (I tested it against my own project), so some guidance on how to do it would be extremely appreciated!

I added that test spec just by looking at the project, but I didn't know what to do with it.

I'll poke you in Slack some time during the day!