Commit 39608d44 highlighted a bug which was never triggered before:
the lastUsed metadata is badly formatted. Indeed, the last_played
method of Grl.Media returns a GLib.DateTime. This date needs to be
formatted into a string in order to be used by MPRIS.
This bug had never been encountered because of the presence of a
try/except block.

Fix the issue by formatting the date into a string in ISO 8601
format.

Closes: #294