Section 91. In an action of slander or libel, if the defendant alleges that the words spoken or published were true, such allegation, although not supported by the evidence, shall not of itself be proof of the malice alleged in the complaint, nor shall statements of the defendant differing in import from those alleged be admissible to establish his malice unless such statements were published in pursuance of a general scheme to defame or otherwise injure the plaintiff. If the plaintiff proposes to introduce evidence of statements of the defendant other than those contained in his pleadings, he shall give the defendant written notice of such intention, specifying the date and content of each such statement, at least fourteen days before trial begins, or earlier if the court so orders; and, if any such statement is introduced in evidence, the defendant shall be permitted to prove that it was true, or was privileged, or any other facts relating thereto which tend to negative malice.