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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 content_type(self):
 # type: () -> str """The request's content-type. Returns: (str):
 The value, if any, of the header 'ContentType' (used by some AWS services) and 'Content-Type'. Otherwise, returns 'application/json' as default. """

        # todo(mvsusp): consider a better default content-type
        return self.headers.get('ContentType') or self.headers.get(
            'Content-Type') or _content_types.JSON