title: Setting title tag's string attribute clears string attribute of another tag in BeautifulSoup
body: <p>I try to change title tags of some html documents using BeautifulSoup with the following code:</p>
  
  <pre><code>&gt;&gt;&gt; doc = BeautifulSoup(open(filename))
  &gt;&gt;&gt; root = doc.find('html') # works only with html parser
  &gt;&gt;&gt; hafta = root.find(id="hafta")
  &gt;&gt;&gt; content = hafta.find('div', {'class':'convHtml'})
  &gt;&gt;&gt; content.find('b').string
  u'BAKANLARA N\u0130\xc7\u0130N KURBAN KES\u0130L\u0130R?'
  &gt;&gt;&gt; doc.title.string = content.find('b').string
  &gt;&gt;&gt; content.find('b').string
  &gt;&gt;&gt;  
  </code></pre>
  
  <p>Oddly, the statement <code>doc.title.string = content.find('b').string</code> clears the string inside <code>content.find('b')</code>. Why does this occur?</p>