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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:

def _get_request_content(self, message=None):
        '''Updates message with default message paramaters.

        :param message: Postmark message data
        :type message: `dict`
        :rtype: JSON encoded `unicode`
        '''
        message = self._cast_message(message=message)
        return message.json()