[google_calendar] fix synchronization and avoid context hacks

Hello,

I have fixed the traceback : Traceback when try to sync second time with google calendar.

More details:

Task : http://www2.odoo.com/web#id=8905&view_type=form&model=project.task&menu_id=3942&action=327

Pad : https://pad.odoo.com/p/openerp-project.task-465VY3INJC

Thanks,
cod-odoo