Allow developers to set the Content-Disposition header in Response::download()

This change allows developers to set the Content-Disposition header in Response::download() without it getting overwritten. This was a necessary change for me because I want to be able to suggest to the browser whether or not to display a PDF or to download it with:

Content-Disposition: inline; filename=file.pdf;

or 

Content-Disposition: attachment; filename=file.pdf;

This change allows me to do that.