[http-server-csharp]: Fix issues with ToDoList spec service generation

[markcowl]

Some issues to correct with ToDoList spec

• toDoItem._dummy: need to preserve serialized name when creating identifiers
• Correct property narrowing in inherited property overrides (userExistsResponse, invalidUserResponse)
  • Make property virtual or abstract in the base type
  • intentionally override property in derived type with 'new' keyword
• Fix handling of anonymous models (toDoPage.pagination)
• Add special case for coalescing nullable type unions (do not represent as object)
• Convert content-type into constraint (toDoItems.create)
• Provide pattern for UserExists error on create and 404 response on get/update/delete
• Add dispatch logic for union requests and helpers for union properties and responses