Frozenstep kotlin download file fix

[Frozenstep]

PR checklist

• [x ] Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• Filed the PR against the correct branch: 3.0.0 branch for changes related to OpenAPI spec 3.0. Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

Description of the PR

This updates api.mustache and ApiClient.kt.mustache to handle when the response from the server is a file. It also makes changes in the generated sample code for kotlin-client-petstore

In addition
-We fixed the default headers so they do not override generated headers
-The response Deserializer now looks at the response header content type instead assuming the response is the same as the accept header in the request.
-We added all accept types into the request header

[HugoMario]

thanks a lot @Frozenstep !!!!!