Unary tuples printed as (T) instead of (T,)

```
$ cat tuple.rs
fn main() {
    let _: uint = (1u,);
}
$ rustc tuple.rs
tuple.rs:2:19: 2:24 error: mismatched types: expected `uint` but found `(uint)` (expected uint but found tuple)
tuple.rs:2     let _: uint = (1u,);
                             ^~~~~
error: aborting due to previous error
```
