LP#1760223: Update HTTP headers to follow standard

LP#1760223: Update HTTP headers to follow standard

In the olden days, some browsers didn't understand the "attachment" content
disposition.  They only understood "inline" but would let the user save a file
if a filename was provided.  However, in these modern days, relevant browsers
all do the right thing and know about both flavors of content disposition, so
we, too, must change with the times and support the letter of the standard.

This commit fixes downloading of MARCXML from the Exporter module.

Signed-off-by: Mike Rylander <mrylander@gmail.com>