_authenticate(dataToSend, done) {
        // use prior sig to construct new string to sign
        const stringToSign = constructChunkStringToSign(this.timestamp,
            this.credentialScope, this.lastSignature, dataToSend);
        this.log.trace('constructed chunk string to sign',
            { stringToSign });
        // once used prior sig to construct string to sign, reassign
        // lastSignature to current signature
        this.lastSignature = this.currentSignature;
        const vaultParams = {
            log: this.log,
            data: {
                accessKey: this.accessKey,
                signatureFromRequest: this.currentSignature,
                region: this.region,
                scopeDate: this.scopeDate,
                stringToSign,
                timestamp: this.timestamp,
                credentialScope: this.credentialScope,
            },
        };
        return vault.authenticateV4Request(vaultParams, null, {}, err => {
            if (err) {
                this.log.trace('err from vault on streaming v4 auth',
                    { error: err, paramsSentToVault: vaultParams.data });
                return done(err);
            }
            return done();
        });
    }
Build the stringToSign and authenticate the chunk
@param {Buffer} dataToSend - chunk sent from _transform or null
if last chunk without data
@param {function} done - callback to _transform
@return {function} executes callback with err if applicable