Bug in generated code

[YalinLi0312]

I found a bug in return type in my recent generated code with the latest autorest in this PR: Azure/azure-sdk-for-python#27205. The return of get_access_policy should be a list of SignedIdentifier (link). The return type defined in swagger is also SignedIdentifiers which is an array of SignedIdentifier.

Please help to fix, thanks

[iscai-msft]

@msyyc @changlong-liu can you guys take a look today, it's blocking tables. For testing, I think we should generate the XML version-tolerant tests with msrest models, that should repro the issue. Then we can fix the issue here. Thanks!