JsonWebToken signed differently in Node 4 & Node 6/8 - node.js

Question

I'm currently generating JWT using node-jsonwebtoken in Node 4. When I'm trying to upgrade to Node 8, I'm unable to sign a JWT generated in Node 4 (using same secret & payload, the signature is different). This is a problem as I don't want to invalidate older tokens.
The problem comes from the node-jwa library, a dependency node-jws which is a dependency of node-jsonwebtoken. I referenced the issue on GitHub, and as you can see the author of the module is able to reproduce the issue. Here is the code if you want to test:
const crypto = require('crypto');
const jwa = require('jwa');
const secret = crypto.createHash('sha256').update('secret').digest('binary');
const payload = {
uid: 'test',
iat: 1455988418,
iss: 'test'
};
const algo = jwa('HS256');
const sig = algo.sign(payload, secret);
console.log(sig);
// Node v4 => "_zPq9vDP4_Ve0mTVTF_9H3NRkluQhoR4yAg8X4yqR8Q"
// Node v6 => "hk9bpxID-HOmvNpJUy7x80KqT5JP8tb_BoAJLYVIYsE"
As I understood, the problem is that the default encoding for digests was changed to utf8 in v5/v6. In v4 and earlier it was binary.
Cf => github.com/nodejs/node/issues/6813
As the maintener of node-jwa is no longer replying, I'm trying to find a workaround as I really don't want to be sticked in Node 4 forever (which is no longer LTS).
I've tried to find where to modify the node-jwa library so that it can sign in Node 8 my token the same way as Node 4 did.
Seems the line affecting my problem is here in the library => github.com/brianloveswords/node-jwa/blob/master/index.js#L35
I've tried to make a few changes, but was unable to make this work...
If you have any idea of a good workaround,
Thank you very much!