Merge #138: Fix crash when searching by date

It seems Xapian returns query descriptions containing non-UTF8 characters
(binary?) when creating queries for dates. This patch uses a default string
when logging the generated query.

Fixes #138

Squashed commit of the following:

commit e5516bd4c0f173848e5455df50208a31c23d35e8
Author: Eric Weikl <eric.weikl@gmx.net>
Date:   Sun Sep 8 13:05:35 2013 +0200

    fix copy&paste

commit 3293a251cad53586bb4d71af0b20e751b8b6e36e
Author: Eric Weikl <eric.weikl@gmx.net>
Date:   Sun Sep 1 17:03:41 2013 +0200

    Fix crash when searching by date

    It seems Xapian returns query descriptions containing non-UTF8 characters
    (binary?) when creating queries for dates. This patch uses a default string
    when logging the generated query.

    Fixes #138