The "correct" way is "wash", but many people's accents won't allow them to say that, especially in the south. It's just one of those things that creeps into a regional accent or dialect that sounds right to them, but wrong to everyone else. I'm not saying that it's wrong or that people in the south are wrong (especially since I'm from Texas), but I'm saying that this is an example of something you will find with just about every different group of English speaking people.