DateTime Format is not Culture Invariant

Description

Serverside expects DateTimeFormat in "dd/MM/yyyy"
As my client system has culture "de-CH", Setting the Clients DateTimeFormat to "dd/MM/yyyy"
Will still lead to formatting the Date in the following Format: "dd.MM.yyyy". The following format string will work: "dd'/'MM'/'yyyy"

See custom format specifier: https://docs.microsoft.com/en-us/dotnet/standard/base-types/custom-date-and-time-format-strings

Client Code Generation: CSharp

Swagger-codegen version

master

Swagger declaration file content or url

Applies to any date

• in: header
  name: bis
  description: >-
  Tag, bis dem die angegebene Aenderung gelten soll. Wenn nicht
  vorhanden gilt die Aenderung bis alle Ewigkeit
  required: false
  type: string
  format: date

Command line used for generation

Steps to reproduce

any date time formatting

Related issues/PRs

None found

Suggest a fix/enhancement

https://github.com/swagger-api/swagger-codegen/blob/2fd38fbbe286980b0c6c7d063da6aa080935cd96/modules/swagger-codegen/src/main/resources/csharp/ApiClient.mustache#L292

Suggestion:
This would in any circumstances lead to the specified format:
return ((DateTime)obj).ToString (Configuration.DateTimeFormat, CultureInfo.InvariantCulture);

Start at modules/swagger-codegen/src/main/resources/csharp/ApiClient.mustache around line 292, where the issue identifies DateTime formatting. Reproduce generation with a de-CH culture and a date format of dd/MM/yyyy; done means the generated C# client preserves the configured separators regardless of the client culture.