Did John Brown, the infamous abolitionist who led the 1859 raid on Harpers Ferry, West Virginia, really take a cross-dressing teenage slave under his wing and name him Onion? No, not at all. “The Good Lord Bird,” which never purports itself as a true retelling, opens each episode with the tagline “All of this is true. Most of it happened.” Usually playing with the past, especially with a history as traumatizing as America’s original sin, slavery, can quickly go left. Just look at “Antebellum.” But creators Ethan Hawke and Mark Richard’s seven-episode Showtime miniseries, based on James McBride’s novel of the same title, just barely glides over the unavoidable traps. Instead, “The Good Lord Bird” is a wickedly funny take on an enigmatic man whose zealotry to the cause of freedom led to the most important event in American history next to the Civil War.