title: cannot replicate model with unique slug field
body: <p>I cannot understand how to replicate model with unique <code>slug</code> field.
  I see success message, but this code doesn't create additional row into DB table. 
  And no messages or exceptions in debugbar.</p>
  
  <pre><code>
      public function handle(Model $model)
      {
          $model-&gt;replicate(['slug']);
          $model-&gt;slug = Str::slug($model-&gt;title, '-') . $model-&gt;id;
          $model-&gt;save();
          return $this-&gt;response()-&gt;success('Скопировано!')-&gt;refresh();
      }
  </code></pre>
  
  <p>And if I add <code>dd($model)</code> somewhere in the middle it doesn't help me, because I don't see anything except Oops message. 
  <a href="https://i.stack.imgur.com/sFuNe.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/sFuNe.jpg" alt="enter image description here"></a>
  Here is migration file</p>
  
  <pre><code>        Schema::create('news_posts', function (Blueprint $table) {
              $table-&gt;increments('id')-&gt;unsigned();
              $table-&gt;integer('category_id')-&gt;unsigned();
              $table-&gt;string('title', 255)-&gt;nullable();
              $table-&gt;string('slug', 255)-&gt;unique();
              $table-&gt;text('fulltext')-&gt;nullable();
              $table-&gt;string('image', 255)-&gt;nullable();
              $table-&gt;boolean('is_published')-&gt;default(false);
              $table-&gt;timestamp('published_at')-&gt;nullable();
              $table-&gt;timestamps();
              $table-&gt;softDeletes();
  
              $table-&gt;foreign('category_id')-&gt;references('id')-&gt;on('news_categories');
              $table-&gt;index('is_published');
          });
  </code></pre>