title: pyyam safe_dump doesn't work to get rid of tags
body: <p>Based on answers to this SO <a href="https://stackoverflow.com/questions/1950306/pyyaml-dumping-without-tags">question</a>, I should use <code>safe_dump</code> instead of <code>dump</code> to get rid of undesired tags like <code>!!python/unicode</code> but it dosn't work for me.</p>
  <p>In my personal case, the undesired tag is <code>!!python/object:module_example.items.ExampleItem</code> and also a dict key <code>_values</code> is added. All of that maybe because I'm using scrapy and my <code>item</code> dict is created through the ExampleItem class.</p>
  <p>Here is the code:</p>
  <pre><code>import yaml
  
  yaml.dump({'obj': item}, outfile)
  # using safe_dump gives an empty output
  </code></pre>
  <p>The current output:</p>
  <pre><code>obj: !!python/object:module_example.items.ExampleItem
    _values:
      first_name: 'foo'
      last_name: 'bar'
  </code></pre>
  <p>The desired output:</p>
  <pre><code>obj:
    first_name: 'foo'
    last_name: 'bar'
  </code></pre>
  <p>How to get rid of those tags ?</p>