I have now reached a use case where I have a class that acts a lot like StreamedResponse but has the purpose of handling the headers, status codes, and outputting of data before the Response object is generated.

The problem is that makeDisposition is not static (in the source there is no reference to any internal variables in makeDisposition in the ResponseHeaderBag) So I see no reason why it cannot be static.
It will also then allows us to use makeDisposition ahead of the response object creation and pass the header along. instead of setting it after creation.