title: Error in Inheritance of class html in osv/fields.py Python/openerp 7?
body: <p>i want to inherit the class html and to redefine the methode _symbol_f(x) which is defined in osv/fields.py in html classe
  the definition of this calss is the following:</p>
  
  <pre><code>class text(_column):
  _type = 'text'
  
  class html(text):
  _type = 'html'
  _symbol_c = '%s'
  def _symbol_f(x):
      if x is None or x == False:
          return None
      return html_sanitize(x)
  
  _symbol_set = (_symbol_c, _symbol_f)
  </code></pre>
  
  <p>i want to redefine the method  _symbol_f(x), so i have created the class wiki_html in the file wiki_html.py like the following</p>
  
  <pre><code>from openerp.osv import fields, osv
  
  class wiki_html(fields.html):
  
  _type = 'html'
  _symbol_c = '%s'
  def _symbol_f(x):
      if x is None or x == False:
          return None
      return html_sanitize_h(x)
  
  _symbol_set = (_symbol_c, _symbol_f)
  </code></pre>
  
  <p>and now i define a field like the following:</p>
  
  <pre><code> 'demandes_description':wiki_html('Description demande'),
  </code></pre>
  
  <p>but i have always this error:</p>
  
  <pre><code>openerp.osv.orm: &lt;class 'openerp.addons.....wiki_html'&gt; type not supported!
  </code></pre>