fix lint

src/utils/merge.js

@@ -1,9 +1,9 @@
 export default function(target, ...sources) {
   for (let i = 0; i < sources.length; i++) {
-    let source = sources[i] || {};
-    for (let prop in source) {
+    const source = sources[i] || {};
+    for (const prop in source) {
       if (source.hasOwnProperty(prop)) {
-        let value = source[prop];
+        const value = source[prop];
         if (value !== undefined) {
           target[prop] = value;
         }