It happens every few weeks or so. If you’re Jewish, it probably happens to you, too. An email arrives from an acquaintance or a relative, invariably including a forwarded message. I click on it, and out pours the worst kind of anti-Semitic dribble, like burning lava seeping across my keyboard. But wait. It’s not anti-Semitism at all. It’s anti-Muslim sentiment. It’s easy to think, at...