def format(self, value):
return value<EOL>
Formats a field's value. No-op by default - field classes that
        modify how the value of existing object keys should be presented should
        override this and apply the appropriate formatting.

        :param value: The value to format
        :exception MarshallingException: In case of formatting problem

        Ex::

            class TitleCase(Raw):
                def format(self, value):
                    return unicode(value).title()