add content-disposition filename to all image responses

[robcast]

Currently we only add a filename via content-disposition for mo=rawfile.

IIIF Image API http://iiif.io/api/image/2.0/#a-implementation-notes proposes to always set a file name via content-disposition.

[robcast]

send-file currently uses the server-side file name which is ok.

send-image does not yet get a file name. I think it should be added when scaling the image. Should the file name also include zoom area and size?