Fix clippy warnings: useless use of format!

[LingMan]

warning: useless use of `format!`
   --> src/pb.rs:358:32
    |
358 |             prefix = prefix + &format!("{}", self.message);
    |                                ^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider using `.to_string()`: `self.message.to_string()`
    |
    = note: `#[warn(clippy::useless_format)]` on by default
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#useless_format

warning: useless use of `format!`
   --> src/pb.rs:440:23
    |
440 |         let mut out = format!("{}", s);
    |                       ^^^^^^^^^^^^^^^^ help: consider using `.to_string()`: `s.to_string()`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#useless_format