base64.decodestring doesn't support buffer

issue2295
review181002
(transplanted from d1efc72e22f98ad9dd5779dcba19c9303be79c2d)

invoice.py

@@ -194,7 +194,7 @@ class Invoice(ModelWorkflow, ModelSQL, ModelView):
                 'FROM "' + self._table + '"')
             for invoice_id, report in cursor.fetchall():
                 if report:
-                    report = base64.decodestring(report)
+                    report = base64.decodestring(str(report))
                     cursor.execute('UPDATE "' + self._table + '" '
                         'SET invoice_report_cache = %s '
                         'WHERE id = %s', (report, invoice_id))