Allow users to customise the data used in the digest

[connorjburton]

In theory the HMAC digest could include any information from the request object in express, for example the domain it was sent to, the port, the protocol etc.

There isn't any real reason to prevent users being able to customise the level data used, the only limitation is what you know on both the client and the server (For example, if you are behind a proxy or load balancer that adds headers that are not known by the client)