fix(h1): "Transfer-Encoding" repair code causes a panic

Closes hyperium#2409

src/headers.rs

@@ -102,9 +102,9 @@ pub fn add_chunked(mut entry: OccupiedEntry<'_, HeaderValue>) {
         // + 2 for ", "
         let new_cap = line.as_bytes().len() + CHUNKED.len() + 2;
         let mut buf = BytesMut::with_capacity(new_cap);
-        buf.copy_from_slice(line.as_bytes());
-        buf.copy_from_slice(b", ");
-        buf.copy_from_slice(CHUNKED.as_bytes());
+        buf.extend_from_slice(line.as_bytes());
+        buf.extend_from_slice(b", ");
+        buf.extend_from_slice(CHUNKED.as_bytes());
 
         *line = HeaderValue::from_maybe_shared(buf.freeze())
             .expect("original header value plus ascii is valid");

tests/client.rs

@@ -401,6 +401,36 @@ test! {
             body: None,
 }
 
+test! {
+    name: client_transfer_encoding_repair,
+
+    server:
+        expected: "\
+            GET / HTTP/1.1\r\n\
+            transfer-encoding: foo, chunked\r\n\
+            host: {addr}\r\n\
+            \r\n\
+            5\r\n\
+            hello\r\n\
+            0\r\n\r\n\
+            ",
+        reply: REPLY_OK,
+
+    client:
+        request: {
+            method: GET,
+            url: "http://{addr}/",
+            headers: {
+                "transfer-encoding" => "foo",
+            },
+            body: "hello", // not Body::empty
+        },
+        response:
+            status: OK,
+            headers: {},
+            body: None,
+}
+
 test! {
     name: client_get_req_body_chunked_http10,