return proxy-authenticate in Proxy auth layer

important... as otherwise browsers fail,
given they seem to rely on that realm info to be sent
prior to it sending credentials

examples/http_connect_proxy.rs

@@ -32,7 +32,7 @@
 //! If you want to see the HTTP traffic in action you can of course also use telnet instead:
 //!
 //! ```sh
-//! telnet 127.0.0.1 40001
+//! telnet 127.0.0.1 62001
 //! ```
 //!
 //! and then type:

src/http/layer/proxy_auth/mod.rs

@@ -130,12 +130,14 @@ where
             } else {
                 Ok(Response::builder()
                     .status(StatusCode::PROXY_AUTHENTICATION_REQUIRED)
+                    .header("Proxy-Authenticate", C::SCHEME)
                     .body(Default::default())
                     .unwrap())
             }
         } else {
             Ok(Response::builder()
                 .status(StatusCode::PROXY_AUTHENTICATION_REQUIRED)
+                .header("Proxy-Authenticate", C::SCHEME)
                 .body(Default::default())
                 .unwrap())
         }