chore(lib): fix nightly unused warning about AsciiExt

src/header/shared/charset.rs

@@ -1,5 +1,6 @@
 use std::fmt::{self, Display};
 use std::str::FromStr;
+#[allow(unused)]
 use std::ascii::AsciiExt;
 
 use self::Charset::*;

src/header/shared/quality_item.rs

@@ -1,3 +1,4 @@
+#[allow(unused)]
 use std::ascii::AsciiExt;
 use std::cmp;
 use std::default::Default;