doc: update deprecated var usage in documentation examples (#7246)

* test(http): fix HTTPS protocol test by using local HTTPS server instead of external request

* docs: update var usage in documentation examples

* docs: updated var to const

---------

Co-authored-by: Jay <jasonsaayman@gmail.com>

README.md

@@ -1104,7 +1104,7 @@ The server will handle it as:
 If your backend body-parser (like `body-parser` of `express.js`) supports nested objects decoding, you will get the same object on the server-side automatically
 
 ```js
-  var app = express();
+  const app = express();
 
   app.use(bodyParser.urlencoded({ extended: true })); // support encoded bodies
 

lib/helpers/spread.js

@@ -7,7 +7,7 @@
  *
  *  ```js
  *  function f(x, y, z) {}
- *  var args = [1, 2, 3];
+ *  const args = [1, 2, 3];
  *  f.apply(null, args);
  *  ```
  *

lib/utils.js

@@ -333,7 +333,7 @@ const isContextDefined = (context) => !isUndefined(context) && context !== _glob
  * Example:
  *
  * ```js
- * var result = merge({foo: 123}, {foo: 456});
+ * const result = merge({foo: 123}, {foo: 456});
  * console.log(result.foo); // outputs 456
  * ```
  *