[gnome-db] Return values of gda_value_*_get

[Gonzalo Paniagua Javier <gonzalo gnome-db org>]

	The question is: should gda_value_*_get() return (for non-basic
	types) a new copy of the value or the value itself?

	The fact is that gda_value_get_string() returns a const pointer
	to the string stored in a GdaValue while gda_value_get_date()
	(which I am modifying right now) returns a GDate created by
	g_date_new().