Build query params string from a dictionary.

        Args:
            params (dict): A dictionary of params

        Returns:
            string: A valid url query params string.

def _build_param_string(params):
        """Build query params string from a dictionary.

        Args:
            params (dict): A dictionary of params

        Returns:
            string: A valid url query params string.
        """
        pairs = []
        for key, value in params.iteritems():
            if value is None:
                value = ''
            pairs.append('{0}={1}'.format(key, value))
        if len(pairs) > 0:
            return '?{0}'.format('&'.join(pairs))
        return ''