utf16_chars() and from_utf16() stop on 0_u16

[SimonSapin]

When the input contains a zero, these functions stop and ignore the rest of the input instead of emitting a perfectly valid U+0000 code point.

Test case:

use std::str;

fn main() {
    let a = "foo\0bar";
    let b = a.to_utf16();
    let c = str::from_utf16(b);
    println!("{:?}", a);
    println!("{:?}", b);
    println!("{:?}", c)
}

Output:

"foo\x00bar"
~[102u16, 111u16, 111u16, 0u16, 98u16, 97u16, 114u16]
~"foo"

Expected output:

"foo\x00bar"
~[102u16, 111u16, 111u16, 0u16, 98u16, 97u16, 114u16]
~"foo\x00bar"