Copy Docker's response code through for raw stream.

This means when Docker says "101 UPGRADED" the proxy will say the
same, instead of "200 OK".  Docker 1.12 barfs if you do that.

proxy/proxy_intercept.go

@@ -78,7 +78,7 @@ func doRawStream(w http.ResponseWriter, resp *http.Response, client *httputil.Cl
 	defer down.Close()
 	defer up.Close()
 
-	if _, err := down.Write([]byte("HTTP/1.1 200 OK\n")); err != nil {
+	if _, err := down.Write([]byte("HTTP/1.1 " + resp.Status + "\n")); err != nil {
 		Log.Warning(err)
 		return
 	}