validateExposeHeaders(headers) {
        if (!headers) {
            return undefined; // indicate ExposeHeaders do not exist
        }
        if (!Array.isArray(headers) || headers.length === 0) {
            return errors.MalformedXML;
        }
        for (let i = 0; i < headers.length; i++) {
            const header = headers[i];
            if (typeof header !== 'string') {
                return errors.MalformedXML;
            }
            if (header.indexOf('*') !== -1) {
                const errMsg = `ExposeHeader ${header} contains a wildcard. ` +
                'Wildcards are currently not supported for ExposeHeader.';
                return errorInstances.InvalidRequest.customizeDescription(errMsg);
            }
            if (!/^[A-Za-z0-9-]*$/.test(header)) {
                const errMsg = `ExposeHeader ${header} contains invalid ` +
                'character.';
                return errorInstances.InvalidRequest.customizeDescription(errMsg);
            }
        }
        return true;
    }
_validator.validateExposeHeaders - check values of ExposeHeader's
@param {string[]} headers - array of ExposeHeader's
@return {(Error|true|undefined)} - Arsenal error on failure, true if
  valid, undefined if optional ExposeHeader's do not exist