'Add a new field element. Each item is required to have a
name.
Arguments:
name  -- The name of the field.'

def add_field(self, name):

    if (name not in self._fields):
        self._fields.add(name)
        field = RequestField(parent=self)
        field['name'] = name
        self.iterables.append(field)
        return field
    return None
