Init session with default or custom headers

        Args:
            headers: A dict of headers (default None, thus using the default
                     header to init the session)

def set_session(self, headers=None):
        """Init session with default or custom headers

        Args:
            headers: A dict of headers (default None, thus using the default
                     header to init the session)
        """
        if headers is None:
            headers = {
                'User-Agent':
                ('Mozilla/5.0 (Macintosh; Intel Mac OS X 10_11_3)'
                 ' AppleWebKit/537.36 (KHTML, like Gecko) '
                 'Chrome/48.0.2564.116 Safari/537.36')
            }
        elif not isinstance(headers, dict):
            raise TypeError('"headers" must be a dict object')

        self.session = Session(self.proxy_pool)
        self.session.headers.update(headers)