[hr_expense] Can not set a default account journal on expense creation

First steps to reproduce :

* activate developper mode
* create a new expense
* go on accouting tab
* select a dedicated account journal (type=purchase)
* through developper option select the option to set default value
* select "force journal" and set it to a default value for everyone
* save
* create a new expense
* look if the default account journal previously set is selected

The accouting journal is not set with the default value...

Going further through code, when trying to set a default value at expense object creation, using _defaults dict doesn't help too, no default value set for journal account...

The only current way to accomplish that is to override the onchange_currency_id method to force it returns an empty {'values': {}} dict.

I suspect that the on_change_currency_id should not use the currency_id in the search domain.