chore(client): place the use of new rust features behind cfg

src/headers.rs

@@ -2,6 +2,8 @@ use bytes::BytesMut;
 use http::HeaderMap;
 use http::header::{CONTENT_LENGTH, TRANSFER_ENCODING};
 use http::header::{HeaderValue, OccupiedEntry, ValueIter};
+#[cfg(not(inherent_ascii))]
+use std::ascii::AsciiExt;
 
 pub fn connection_keep_alive(value: &HeaderValue) -> bool {
     connection_has(value, "keep-alive")