fix: frontend check `is_http_backend` bug

pywebio/html/index.html

@@ -73,8 +73,6 @@
     *   is_http_backend('http://localhost:8080/io').then(function(http_backend){ });
     * */
     function is_http_backend(backend_addr) {
-        if (!backend_addr.startsWith('http://') && !backend_addr.startsWith('https://'))
-            backend_addr = 'http://' + backend_addr;
         return new Promise(function (resolve, reject) {
             $.get(backend_addr, {test: 1}, 'html').done(function (data) {
                 resolve(data === 'ok');
@@ -106,10 +104,10 @@
     var md_body = $('#markdown-body');
 
     const debug = getParameterByName('_pywebio_debug');
-    is_http_backend('http://localhost:8080/io').then(function (http_backend) {
+    is_http_backend('./io').then(function (http_backend) {
         var session;
         if (http_backend)
-            session = new WebIO.HttpWebIOSession('/io');
+            session = new WebIO.HttpWebIOSession('./io');
         else
             session = new WebIO.WebSocketWebIOSession(get_ws_addr());
         var ctrl = new WebIO.WebIOController(session, md_body, $('#input-container'));