“… In the ordinary and normal case [the judge] may feel that if the credit of the prosecutor or his witnesses has been attacked, it is only fair that the jury should have before them material on which they can form their judgment whether the accused person is any more worthy to be believed than those he has attacked. It is obviously unfair that the jury should be left in the dark about an accused person's character if the conduct of his defence has attacked the character of the prosecutor or the witnesses for the prosecution within the meaning of the section.”