actix-web: InternalError can trigger memory unsafety (such as use-after-free or double-free)

There is an unsafe impl Send<T> for InternalError<T>, making it so one can construct an InternalError with any type, and accidentally send it to another thread. However, the implementation of InternalError isn’t actually safe to implement Send for any T.

Consider this example:

let my_err = Rc::new("foobar".to_owned());
let int_err = InternalError::new(my_err.clone(), StatusCode::BAD_REQUEST);
thread::spawn(move || {
    let _moved = int_err;
    // clone 1 dropped
})
let my_err2 = my_err.clone();
// clone 2 dropped
// orig dropped

The drop of the InternalError in the other thread can cause the ref count to race with the clone in the first thread. It’s now possible that the 2nd drop (out of 3) will free the allocation. Using the 3rd (the original) afterwards would cause use-after-free bugs. Finally dropping it would cause a double-free.

Seems related to #289