[FIX] account: using wizard methods instead of invoice methods

Typo during port of account.invoice to new API.

(Rebase of 02a36b6 for odoo#1052)

addons/account/account_invoice.py

@@ -145,7 +145,7 @@ def _compute_residual(self):
             else:
                 self.residual = new_value
         # prevent the residual amount on the invoice to be less than 0
-        self.residual = max(self.residual, 0.0)            
+        self.residual = max(self.residual, 0.0)
 
     @api.one
     @api.depends(
@@ -1619,8 +1619,8 @@ def send_mail(self):
                 context.get('default_res_id') and context.get('mark_invoice_as_sent'):
             invoice = self.env['account.invoice'].browse(context['default_res_id'])
             invoice = invoice.with_context(mail_post_autofollow=True)
-            self.write({'sent': True})
-            self.message_post(body=_("Invoice sent"))
+            invoice.write({'sent': True})
+            invoice.message_post(body=_("Invoice sent"))
         return super(mail_compose_message, self).send_mail()
 
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