panic: byte index 12 is not a char boundary

[Turbo87]

panic: byte index 12 is not a char boundary; it is inside '打' (bytes 10..13) of `crates/实打实打算/d's'd`
  File "panicking.rs", line 515, in rust_begin_unwind
  ?, in route_recognizer::nfa::Thread::extract
  ?, in route_recognizer::nfa::NFA<T>::process
  ?, in route_recognizer::Router<T>::recognize
  ?, in conduit_router::RouteBuilder::call
...
(32 additional frame(s) were not displayed)

this can be reproduced by:

#[test]
fn test_chinese() {
    let mut router = Router::new();
    router.add("/crates/:foo/:bar", "Hello".to_string());

    let m = router.recognize("/crates/实打实打算/d's'd").unwrap();
    assert_eq!(m.handler().as_str(), "Hello");
    assert_eq!(m.params().find("foo"), Some("实打实打算"));
    assert_eq!(m.params().find("bar"), Some("d's'd"));
}