fix #1671 (#1678)

admin/write-post.php

@@ -98,7 +98,7 @@ $post = \Widget\Contents\Post\Edit::alloc()->prepare();
                             <label class="typecho-label"><?php _e('分类'); ?></label>
                             <?php \Widget\Metas\Category\Rows::alloc()->to($category); ?>
                             <ul>
-                                <?php $categories = $post->categories->toArray('mid'); ?>
+                                <?php $categories = array_column($post->categories, 'mid'); ?>
                                 <?php while ($category->next()): ?>
                                     <li><?php echo str_repeat('&nbsp;&nbsp;&nbsp;&nbsp;', $category->levels); ?><input
                                             type="checkbox" id="category-<?php $category->mid(); ?>"