[simple-scan] Remove warning when saving to pdf

[Robert Ancell <rancell src gnome org>]

commit c69f8531b0fe960e16712b08dcdc80a44145552d
Author: Stéphane Fillion <stphanef3724 gmail com>
Date:   Fri May 19 22:04:18 2017 -0400

    Remove warning when saving to pdf

 src/book.vala |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/src/book.vala b/src/book.vala
index c27b60d..8edb9d8 100644
--- a/src/book.vala
+++ b/src/book.vala
@@ -197,8 +197,8 @@ public class Book
     private uint8[] compress_jpeg (Gdk.Pixbuf image, int quality, int dpi)
     {
         jpeg_data = new ByteArray ();
-        string[] keys = { "quality", "density-unit", "x-density", "y-density", null };
-        string[] values = { "%d".printf (quality), "dots-per-inch", "%d".printf (dpi), "%d".printf (dpi), 
null };
+        string[] keys = { "quality", "x-dpi", "y-dpi", null };
+        string[] values = { "%d".printf (quality), "%d".printf (dpi), "%d".printf (dpi), null };
         try
         {
             image.save_to_callbackv (write_pixbuf_data, "jpeg", keys, values);