Fixes for correctly matching all double values in updates
The gist of the problem is described at https://bugzilla.gnome.org/show_bug.cgi?id=659600#c1.
Doing DELETE WHERE { <foo> example:doubleProperty ?v }
fails for certain double values, because the value obtained when finding the solutions to the WHERE clause has a lower precision, making the value not look equal when trying to delete it. This resulted in the value not being actually deleted.
Fix this by ensuring we get a full precision double value, so the sanity check comes out sane.