Fix compile warnings

Warnings before the fix:

warning: trait objects without an explicit `dyn` are deprecated
   --> src/lib.rs:677:32
    |
677 |     fn cause(&self) -> Option<&Error> {
    |                                ^^^^^ help: use `dyn`: `dyn Error`
    |
    = note: `#[warn(bare_trait_objects)]` on by default

warning: variable does not need to be mutable
   --> src/lib.rs:260:21
    |
260 |                 for mut request in server.incoming_requests() {
    |                     ----^^^^^^^
    |                     |
    |                     help: remove this `mut`
    |
    = note: `#[warn(unused_mut)]` on by default