[C#] fix issue with API key (query)

[wing328]

PR checklist

• 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: master for non-breaking changes and 3.0.0 branch for breaking (non-backward compatible) changes.

Description of the PR

• Fixed the issue with ParameterToKeyValuePairs for query parameters
• Updated petstore-with-fake-endpoints-models-for-testing.yaml with additional test cases to cover the issue moving forward