Where the cause of divorce consists of a course of offensive conduct, or arises in cases of cruelty from excessive acts of ill-treatment, which may aggregately constitute the offense, cohabitation, or passive endurance, or conjugal kindness shall not be evidence of condonation of any of the acts constituting such cause, unless accompanied by an express agreement to condone. In such cases, condonation can be made only after the cause of divorce has become complete, as to the acts complained of.