Upgrading desktop-file-utils on fresh 18.1 MATE installation removes xreader as default PDF viewer

[Kerrigan Joseph]

1) Linux Mint 18.1 MATE Edition
2) Immediately after installing, ran apt-get clean/update/upgrade, which
   upgraded (among other things) desktop-file-utils from version 0.22-1ubuntu5
   to version 0.22-1ubuntu5.1
3) The entry in /usr/share/applications/defaults.list for application/pdf changed from
   application/pdf=xreader.desktop;evince.desktop;atril.desktop
   to
   application/pdf=evince.desktop
   By watching the output of grep pdf /usr/share/applications/defaults.list as the
   upgrade was occurring, I could see that the file changed while desktop-file-utils
   was being installed. Since evince isn't installed by default, it seems to default
   to opening PDFs with GIMP after xreader is removed from the list of default applications.
4) I expected xreader to continue to be the default PDF app
5) It appears to happen every time but it's not very convenient to repeatedly reinstall my
   operating system, so I've only tested it twice.