[BUG] [cpp-restsdk] Cannot generate "accept header" in client sdk correctly.

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
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Description

Use sample yaml to reproduce the issue:

We defined a new accept header "application/ABC+json:" to replace old "application/json:" in response content type
(paths: /pets/post/responses/'200'/content/application/ABC+json).
For cpp-restsdk, it does not work.
For csharp-netcore, it works well.

openapi-generator version

5.3.0

OpenAPI declaration file content or url

Here is the yaml file (petstore.yaml): (Line 28 for application/ABC+json)
https://gist.github.com/oneiric/1ded544a13b561d9a586789b5919e5a7

Generation Details

Steps to reproduce

Wrong results in cpp-restsdk output:

openapi-generator-cli generate -i petstore.yaml  -g cpp-restsdk  -o ./CppClientSDK 

See line 57~ line 78 in PetApi.cpp, it always throws exceptions. And new accept type "application/ABC+json" is not handled.
https://gist.github.com/oneiric/e17ef85c02007abe5c61ecd652091162

Correct results in csharp-netcore output:

openapi-generator-cli generate -i petstore.yaml  -g csharp-netcore  -o ./CSharpClientSDK 

See line 295 ~ line 304 in PetApi.cs, new accept type "application/ABC+json" can work well.
https://gist.github.com/oneiric/f216527a08300bfb3b67dd7494c02f72

Expected output:
cpp-resksdk can work well like csharp.

Related issues/PRs

Suggest a fix

Use the linked petstore.yaml and run the cpp-restsdk generation command first, then inspect the generated PetApi.cpp around lines 57-78. Compare its handling of the application/ABC+json accept type with the csharp-netcore output in PetApi.cs around lines 295-304. Done means the generated C++ client handles the custom accept type without always throwing exceptions.