Re-open file in utils.open_image() if it is closed

This is a workaround for a Django issue described here:  https://code.djangoproject.com/ticket/13750

The specific scenario I encountered is using an `ImageSpecField` from ImageKit on an `ImageField` that has `width_field` & `height_field` params.  Django sets the width/height fields before ImagetKit/PILKit gets to the file, and by that point it's closed and there's an I/O error.