Decompiler Bug: do a better job of sprinting None values

```
Sprinting direct union case constructions like <@ None : option<int> @> works 
fine, but e.g.

let f () = None:option<int>
unquote <@ f () @>

outputs

f()
null

when it should output

f()
None
```

Original issue reported on code.google.com by `stephen....@gmail.com` on 11 Apr 2012 at 6:44
