[FIX] account_voucher: validation of refund in multicurrency

Fixing several issues for refunds in multicurrency mode that prevented the moves to be balanced.
The amount_currency needs to use the absolute value, as a refund will have a negative amount.
The sign for currency_rate_difference needs to use the line instead of the voucher as they can have different value. e.g. a voucher of 1000$ with invoice of 1200$ and refund of 200$ will have two lines and their currency_rate_difference should have different signs.
Avoids doubling the value in foreign_currency_diff
Fixes #1490, opw 607118 & 611580

addons/account_voucher/account_voucher.py

@@ -1055,7 +1055,8 @@ class account_voucher(osv.osv):
                 'period_id': voucher.period_id.id,
                 'partner_id': voucher.partner_id.id,
                 'currency_id': company_currency <> current_currency and  current_currency or False,
-                'amount_currency': company_currency <> current_currency and sign * voucher.amount or 0.0,
+                'amount_currency': (sign * abs(voucher.amount) # amount < 0 for refunds
+                    if company_currency != current_currency else 0.0),
                 'date': voucher.date,
                 'date_maturity': voucher.date_due
             }
@@ -1217,7 +1218,7 @@ class account_voucher(osv.osv):
             if line.amount == line.amount_unreconciled:
                 if not line.move_line_id:
                     raise osv.except_osv(_('Wrong voucher line'),_("The invoice you are willing to pay is not valid anymore."))
-                sign = voucher.type in ('payment', 'purchase') and -1 or 1
+                sign = line.type =='dr' and -1 or 1
                 currency_rate_difference = sign * (line.move_line_id.amount_residual - amount)
             else:
                 currency_rate_difference = 0.0
@@ -1275,8 +1276,7 @@ class account_voucher(osv.osv):
                         # otherwise we use the rates of the system
                         amount_currency = currency_obj.compute(cr, uid, company_currency, line.move_line_id.currency_id.id, move_line['debit']-move_line['credit'], context=ctx)
                 if line.amount == line.amount_unreconciled:
-                    sign = voucher.type in ('payment', 'purchase') and -1 or 1
-                    foreign_currency_diff = sign * line.move_line_id.amount_residual_currency + amount_currency
+                    foreign_currency_diff = line.move_line_id.amount_residual_currency - abs(amount_currency)
 
             move_line['amount_currency'] = amount_currency
             voucher_line = move_line_obj.create(cr, uid, move_line)