chore(lib): fix nightly unused warning about AsciiExt

src/header/common/link.rs

@@ -1,6 +1,7 @@
 use std::fmt;
 use std::borrow::Cow;
 use std::str::FromStr;
+#[allow(unused)]
 use std::ascii::AsciiExt;
 
 use mime::Mime;