[http-server-csharp] literal type is not properly generated

[ArcturusZhang]

In the contoso project, we have this interface generated:

public interface IWidgets
{
	Task<WidgetSuite> CreateOrUpdateWidgetAsync(string apiVersion, string widgetName, "application/merge-patch+json" contentType, WidgetSuite resource);
}

It misuses a string value as the type of contentType which is incorrect.
This might be indicating the generator is not properly handling the literal types.