Implementing my own on_debug for the engine is broken due to incorrect handling of the message as a debug string, not the string given to the debug call

To Reproduce:

1. Implement your own debug function (only the text value given is relevant for this issue):

engine.on_debug(move |text, _, _| {
    println!("DEBUG: {}", text);
});

2. Call debug from Rhai

debug("test");

3. Observe that the printed line is NOT the given string, but instead its std::fmt::Debug implementation, despite the print within our debug function specifying std::fmt::Display. We get:

DEBUG: "test"

when we should in fact get:

DEBUG: test

Can debug be changed so that it's given the actual string passed to it, as print is?