canvas.bbox() should return a tuple containing values whether an item is hidden or not. canvax.coords() does return a tuple when an item is hidden.

Steps to reproduce:

```
  from tkinter import * 
  root = Tk()
  canvas = Canvas(root)
  id1 = canvas.create_line(10,5,20,5, tags='tunnel')
  id2 = canvas.create_line(10,8,20,8, tags='tunnel')
  canvas.bbox('tunnel')   # return a tupple
  canvas.itemconfig('tunnel', state='hidden')
  canvas.bbox('tunnel')   # return nothing not even None
```

I need bbox to return a tuple containing values. The consequences is that the code must make the items temporarily visible before it can invoke the bbox function. This turning on and off creates flashing items in my program.

Thanks in advance!