Where any actual waiver by a lessor or the persons deriving title under him of the benefit of any covenant or condition in any lease is proved to have taken place in any particular instance, such waiver shall not be deemed to extend to any instance, or to any breach of covenant or condition save that to which such waiver specially relates, nor operate as a general waiver of the benefit of any such covenant or condition.

This section applies unless a contrary intention appears and extends to waivers effected after the twenty-third day of July, eighteen hundred and sixty.