Refactor createETagGenerator for improved readability and modern syntax

- Replaced function declaration with arrow function
- Separated ternary condition into multiple lines for clarity
- Changed `var` to `const` for better scoping and modern standards

lib/utils.js

@@ -226,14 +226,14 @@ exports.setCharset = function setCharset(type, charset) {
  * @private
  */
 
-function createETagGenerator (options) {
-  return function generateETag (body, encoding) {
-    var buf = !Buffer.isBuffer(body)
-      ? Buffer.from(body, encoding)
-      : body
-
-    return etag(buf, options)
-  }
+function createETagGenerator(options) {
+  return (body, encoding) => {
+    const buf = Buffer.isBuffer(body)
+      ? body
+      : Buffer.from(body, encoding);
+
+    return etag(buf, options);
+  };
 }
 
 /**