This is a very common myth. There are no ill effects of feeding your snake in it's enclosure. The only time it would cause the snake to be aggressive is if the *only* time you ever opened the enclosure was to feed it. Say you get the snake out twice a week, and you check it's water once or twice a week. Then you feed it once a week. Only 1 out of 5 times you open the enclosure are you putting food in there. Your snake isn't going to mistake you for food.
There are some people that feed their snakes on a very regular basis, and a snake can get used to that. If you do that then on feeding day each week, the snake is more likely to be aggressive, but from what I understand it can help get a stronger feeding response. And, if you have a set feeding day, you know not to handle your snake on that day, and there are no problems. ^_^