[FIX] account* modules: added 'account_period_prefer_normal':True in context when...

[FIX] account* modules: added 'account_period_prefer_normal':True in context when trying to find the period related to a date.

This patch will be forward ported in trunk by changing the behaviour of account_period.find() in order to fetch the normal periods by default (account_period_prefer_normal will be True by default) because there are no business case i could think of where you'd like to get the opening period (except in the closure but it's held in a different way there). On the other hand, it's pretty easy to forget to put that key in the context and introduce a new bug that will select the opening period instead of the wanted one

bzr revid: qdp-launchpad@openerp.com-20130418102433-t52uj23trkpr8vnb