Use string assignment operator instead of concat()

The assignment operator is a lot faster.

core/websock.js

@@ -105,7 +105,7 @@ Websock.prototype = {
         // Handle large arrays in steps to avoid long strings on the stack
         for (let i = 0; i < len; i += 4096) {
             let part = arr.slice(i, Math.min(i + 4096, len));
-            str = str.concat(String.fromCharCode.apply(null, part));
+            str += String.fromCharCode.apply(null, part);
         }
         return str;
     },