Header gets normalized when using req.Header.Set

Setting headers via req.Header.Set causes header names to get normalized (e.g. Sec-WebSocket-Key -> Sec-Websocket-Key).
With gorilla/websocket, the same server accepts the connection without issues, but using this library the handshake fails.

After debugging, I found that the server was specifically expecting Sec-WebSocket-Key, while the client sent the normalized name.
Servers should ideally treat headers case-insensitively, but the normalization might be unintentional, so I wanted to report it.