Pull Request #374 has broken the use of the 'none' algorithm.

Noticed this in my own test cases - when using the none algorithm the sign() function now throws an error if a secret isn't defined. (which is find for `alg: none`)

I believe the simple workaround which preserves the workaround added in #374 is to simply set the secretOrPrivateKey to any string when the algorithm is set to `none`. Will add a PR shortly.