calendar: Fix attribute escaping for GDataCalendarCalendar
calendar: Fix attribute escaping for GDataCalendarCalendar

Helps: bgo#631033

GNOME/libgdata,pwithnall/libgdata,pwithnall/libgdata,Distrotech/libgdata,Pecisk/libgdata,Pecisk/libgdata,Pecisk/libgdata-gtasks,Pecisk/libgdata-gtasks,Pecisk/libgdata,Distrotech/libgdata,Distrotech/libgdata,Pecisk/libgdata-gtasks