Improve Kotlin code quality

[kroegerama]

Some minor improvements to the kotlin-client template. Use val instead of var where possible. Use .apply{} combined with the builder pattern. Use map[key] = value instead of map.put(key, value).

@jimschubert @dr4ke616 @karismann @Zomzog

[jimschubert]

@kroegerama This looks good, thanks for the improvement!

Can you regenerate the kotlin samples to allow CircleCI to pass?

Those scripts are (and require that you've done mvn install beforehand):

bin/kotlin-client-petstore.sh
bin/kotlin-client-string.sh
bin/kotlin-client-threetenbp.sh

[wing328]

@kroegerama thanks for the PR. For upcoming PRs, I would suggest creating a new branch as per Git best practice.

About Kotlin code format, you can also use post-processing file hook to format the Kotlin file using your favorite code formatter. Let me know if you need more information on that.

[wing328]

@kroegerama thanks again for your contribution, which has been included in the v4.0.1 release (https://twitter.com/oas_generator/status/1135534738658062336)