If no protocol specified, fallback to req.protocol, not to https://

[rubenstolk]

If I only specify a path property and not the callbackUrl, I would expect the protocol to fallback to whatever is used in the request, not https.

For example, when I have my server running on http://localhost:4000 and I specify only the property 'path' to be /callback, I would expect the callbackUrl to be http://localhost:4000/callback, currently it will be https://localhost:4000/callback.

[ploer]

Yup, that makes sense to me. Care to put together a pull request?