ACRE-1824
ACRE-1824

Have acre.response.set_header() actually set the header.  Move current behavior to acre.response.add_header().
googlearchive/acre,freebase/acre,googlearchive/acre,freebase/acre,freebase/acre,freebase/acre,freebase/acre,freebase/acre,googlearchive/acre,googlearchive/acre,googlearchive/acre,googlearchive/acre