#fmt handles %f incorrectly when the number needs rounding (truncates instead)

Looks like %f always truncates instead of rounding to the desired precision.  For example, this:

use std;
fn main() {
  let f = 5.1;
  std::io::println(#fmt("hello world %f!", f));
} 

prints out "hello world 5.099999!" instead of the expected "hello world 5.100000!".
