I haven't looked at the patch yet, but I think the right approach will be to extend StreamResponse with a new method:
void prepareResponse(Response response);
And make sure Response includes the necessary setHeader() methods. This will give the StreamResponse implementation the chance to set any headers it wants before the bytestream is sent down.
To some degree, having the getContentType() method is uncessary (the SR impl could set the content type), but since its so likely that an implementation wouldn't otherwise know to set the content type, I feel it is worth leaving unchanged.