title: Strip html tags - lxml.html.clean.clean_html doesn't work as expected
body: <p>I want to strip all html tags from a string except some I specify.
  If I call the constructor with default values everything works fine:</p>
  
  <pre><code>&gt;&gt;&gt; cleaner = lxml.html.clean.Cleaner()
  &gt;&gt;&gt; cleaner.clean_html('''&lt;i&gt;italic&lt;/i&gt;&lt;script&gt;alert('');&lt;/script&gt;''')
  '&lt;span&gt;&lt;i&gt;italic&lt;/i&gt;&lt;/span&gt;'
  </code></pre>
  
  <p>But when I try to specify some tags, things doesn't work anymore:</p>
  
  <pre><code>&gt;&gt;&gt; allowed_tags = ['i','s']
  &gt;&gt;&gt; cleaner = lxml.html.clean.Cleaner(remove_unknown_tags=False,allow_tags=allowed_tags)
  &gt;&gt;&gt; cleaner.clean_html('''&lt;i&gt;italic&lt;/i&gt;&lt;s&gt;strike&lt;/s&gt;''')
  '&lt;span&gt;&lt;/span&gt;'
  </code></pre>
  
  <p>So what am i doing wrong?</p>