Export generateHeaderString or similar testing utility

[tatethurston]

When writing an automated test case to test processing a Stripe webhook, I needed to copy the generateHeaderString function (found here:

stripe-node/test/Webhook.spec.js

Line 140 in 57d5f09

function generateHeaderString(opts) {

).

It would be great to provide this as an exported module, and include a brief example in the readme using it to write tests against a webhook endpoint.

[paulasjes-stripe]

Hi there, this should be exposed and ready for use in the next release!