title: ActiveAdmin won't save has many and belongs to many field
body: <p>I have 2 models. Category and Post. They are connected using a has_many_and_belongs_to_many relationship. I checked in the rails console and the relationship works. </p>
  
  <p>I created checkboxes in activeadmin to set the post categories using this form field:</p>
  
  <pre><code>f.input :categories, as: :check_boxes, collection: Category.all
  </code></pre>
  
  <p>The problem is when I try to save it because every other field data (title, body, meta infos etc.) is saved, but the category stays the same even if I unchecked it, or checked another too.</p>
  
  <p>I am using strong parameters like this:</p>
  
  <pre><code>post_params = params.require(:post).permit(:title,:body,:meta_keywords,:meta_description,:excerpt,:image,:categories)
  </code></pre>
  
  <p>Please give me some suggestions to make active admin save the categories too!</p>
  
  <p>Best Wishes,
  Matt</p>