fix: use isPlainObject internally

src/object.ts

@@ -1,6 +1,12 @@
 import { objectify } from './array'
 import { toInt } from './number'
-import { isArray, isIntString, isObject, isPrimitive } from './typed'
+import {
+  isArray,
+  isIntString,
+  isObject,
+  isPlainObject,
+  isPrimitive
+} from './typed'
 
 const hasOwnProperty = /* @__PURE__ */ Function.prototype.call.bind(
   Object.prototype.hasOwnProperty
@@ -278,7 +284,7 @@ export const assign = <X extends Record<string | symbol | number, any>>(
   const merged = { ...initial }
   for (const key in override) {
     if (hasOwnProperty(override, key)) {
-      merged[key] = isObject(initial[key])
+      merged[key] = isPlainObject(initial[key])
         ? assign(initial[key], override[key])
         : override[key]
     }
@@ -297,7 +303,7 @@ export const assign = <X extends Record<string | symbol | number, any>>(
 export const keys = <TValue extends object>(value: TValue): string[] => {
   if (!value) return []
   const getKeys = (nested: any, paths: string[]): string[] => {
-    if (isObject(nested)) {
+    if (isPlainObject(nested)) {
       return Object.entries(nested).flatMap(([k, v]) =>
         getKeys(v, [...paths, k])
       )