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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 _http_req_apply_default_headers(self, request_headers, content_type, body):
 """Set default values for common HTTP request headers :param dict request_headers: The HTTP request headers :param content_type: The mime-type used in the request/response :type content_type: :py:class:`ietfparse.datastructures.ContentType` or str :param mixed body: The request body :rtype: dict """

        if not request_headers:
            request_headers = {}
        request_headers.setdefault(
            'Accept', ', '.join([str(ct) for ct in AVAILABLE_CONTENT_TYPES]))
        if body:
            request_headers.setdefault(
                'Content-Type', str(content_type) or str(CONTENT_TYPE_MSGPACK))
        if hasattr(self, 'correlation_id'):
            request_headers.setdefault(
                'Correlation-Id', self.correlation_id)
        elif hasattr(self, 'request') and \
                self.request.headers.get('Correlation-Id'):
            request_headers.setdefault(
                'Correlation-Id', self.request.headers['Correlation-Id'])
        return request_headers