Test Add/Update canonical_url
==================

    >>> self.loginAsManager()
    >>> response = self.publish('/plone/portal_pingtool/manage_overview', self.basic_auth)
    >>> 'value="Add"' in  response.getBody()
    True
    >>> self.failUnless(self.portal.getProperty('canonical_url', None) is None)
    >>> self.portal.manage_addProperty('canonical_url', 'localhost', 'string')
    >>> self.failUnless(self.portal.getProperty('canonical_url', None) == 'localhost')
    >>> response = self.publish('/plone/portal_pingtool/manage_overview', self.basic_auth)
    >>> 'value="Add"' in  response.getBody()
    False
    >>> 'value="Update"' in  response.getBody()
    True
    >>> self.portal.manage_changeProperties(canonical_url='')
    >>> response = self.publish('/plone/portal_pingtool/manage_overview', self.basic_auth)
    >>> 'value="Add"' in  response.getBody()
    False
    >>> 'value="Update"' in  response.getBody()
    True