Only allocate error message if the error occurred

Suggested by clippy:

warning: use of `ok_or` followed by a function call
  --> src/handshake/server.rs:20:19
   |
20 |           let key = self.headers.find_first("Sec-WebSocket-Key")
   |  ___________________^ starting here...
21 | |             .ok_or(Error::Protocol("Missing Sec-WebSocket-Key".into()))?;
   | |_______________________________________________________________________^ ...ending here
   |
   = note: #[warn(or_fun_call)] on by default
help: try this
   |         let key = self.headers.find_first("Sec-WebSocket-Key").ok_or_else(|| Error::Protocol("Missing Sec-WebSocket-Key".into()))?;
   = help: for further information visit https://github.com/Manishearth/rust-clippy/wiki#or_fun_call

src/client.rs

@@ -73,7 +73,7 @@ fn wrap_stream(stream: TcpStream, _domain: &str, mode: Mode) -> Result<AutoStrea
 fn connect_to_some<A>(addrs: A, url: &Url, mode: Mode) -> Result<AutoStream>
     where A: Iterator<Item=SocketAddr>
 {
-    let domain = url.host_str().ok_or(Error::Url("No host name in the URL".into()))?;
+    let domain = url.host_str().ok_or_else(|| Error::Url("No host name in the URL".into()))?;
     for addr in addrs {
         debug!("Trying to contact {} at {}...", url, addr);
         if let Ok(raw_stream) = TcpStream::connect(addr) {

src/handshake/server.rs

@@ -18,7 +18,7 @@ impl Request {
     /// Reply to the response.
     pub fn reply(&self) -> Result<Vec<u8>> {
         let key = self.headers.find_first("Sec-WebSocket-Key")
-            .ok_or(Error::Protocol("Missing Sec-WebSocket-Key".into()))?;
+            .ok_or_else(|| Error::Protocol("Missing Sec-WebSocket-Key".into()))?;
         let reply = format!("\
         HTTP/1.1 101 Switching Protocols\r\n\
         Connection: Upgrade\r\n\