If you have set up your UIBarButtonItem with an image in the storyboard, one small hack to change the renderingMode is to add the following code to your viewDidLoad(). This way you don't have to resort to adding the entire button and image in code.
if let navButton = self.navigationItem.leftBarButtonItem, let buttonImage = navButton.image {
navButton.image = buttonImage.withRenderingMode(.alwaysOriginal)
}