title: prevent automatically append a relation when using accessor
body: <p>Suppose we have a <code>Product</code> model like this :</p>
  
  <pre><code>class Product extends Model
  {
      protected $appends = ['picture'];
  
      public function getPictureAttribute()
      {
          $picture = NULL;
          if (!$this-&gt;images-&gt;isEmpty()) {
              $picture = $this-&gt;images-&gt;where('selected', TRUE)-&gt;first()-&gt;path;
          }
          return $picture;
      }
  
      public function images()
      {
          return $this-&gt;morphMany(Image::class, 'imageable');
      }
  }
  </code></pre>
  
  <p>In the other hand there is an <code>Image</code> model like this : </p>
  
  <pre><code>class Image extends Model
  {
      protected $fillable = ['title', 'path', 'imageable_id', 'imageable_type', 'selected'];
  
      public function imageable()
      {
          return $this-&gt;morphTo();
      }
  }
  </code></pre>
  
  <p>As you can see each <strong>product</strong> can have many <strong>images</strong>. one of those images can be <strong>selected</strong> as default and now I want when fetch a <code>Product</code>,that be  include a <code>picture</code> field that holds <code>path</code> of that selected image. for this I appended a <code>picture</code>  attribute to <code>Product</code> model as an accessor.</p>
  
  <p>But each time I call a specific product for example in <code>show</code> controller method like this : </p>
  
  <pre><code>public function show(Product $product)
  {
      return $product;
  }
  </code></pre>
  
  <p>Result is like this (a <code>images</code> property is included automatically ) : </p>
  
  <pre><code>{
      "product_id": 1,
      "code": null,
      "created_at": "2017-12-11 12:21:49",
      "updated_at": "2018-01-23 09:38:38",
      "deleted_at": null,
      "picture": "path 2",
      "images": [
          {
              "id": 9,
              "title": "salam",
              "path": "path 1",
              "imageable_id": 1,
              "imageable_type": "Modules\\Product\\Entities\\Product",
              "selected": 0,
              "created_at": "2018-01-23 09:38:38",
              "updated_at": "2018-01-23 09:38:38"
          },
          {
              "id": 10,
              "title": "in the name of god",
              "path": "path 2",
              "imageable_id": 1,
              "imageable_type": "Modules\\Product\\Entities\\Product",
              "selected": 1,
              "created_at": "2018-01-23 09:38:38",
              "updated_at": "2018-01-23 09:38:38"
          }
      ],
      "title": "Updataed Title"
  }
  </code></pre>
  
  <p>I don't know what is problem and how can I solve that. </p>