title: is_single() function dose not show thumbnail image
body: <p>I am using <code>get_template_part('content');</code> in <code>single.php</code>. The <code>the_post_thumbnail('large-thumbnail');</code> function shows the thumbnail image perfectly before. But it does not show with <code>is_single()</code> function in <code>content.php</code>.</p>
  
  <p>content.php code:</p>
  
  <pre><code>&lt;?php
  if(is_single()){
  the_post_thumbnail('large-thumbnail');
  the_content();
  }else{
  the_post_thumbnail('small-thumbnail');
  echo get_the_excerpt();
  ?&gt;
  &lt;a href="&lt;?php the_permalink(); ?&gt;"&gt;Read more&amp;raquo;&lt;/a&gt;
  &lt;?php } ?&gt;
  </code></pre>
  
  <p>single.php: </p>
  
  <pre><code>&lt;article&gt;
          &lt;div class="single_post_content"&gt;
              &lt;h5&gt;&lt;?php the_title(); ?&gt;&lt;/h5&gt;
  
              &lt;?php
              get_template_part('content');
              ?&gt;
          &lt;/div&gt;
      &lt;/article&gt;
  </code></pre>