-Infinity, Infinity and NaN do not fail "nbf" and "exp" claim validation

[MitMaro]

This was an issue found during the work I did for #497.

What

Passing -Infinity, Infinity and NaN for nbf in the payload is not caught by the claim validation. Since these values all JSON.stringify to null, they should be treated similar to passing null to nbf.

Test Case

const jwt = require('jsonwebtoken');
console.log(jwt.sign({nbf: Infinity}, undefined, {algorithm: 'none'}));
console.log(jwt.sign({nbf: -Infinity}, undefined, {algorithm: 'none'}));
console.log(jwt.sign({nbf: NaN}, undefined, {algorithm: 'none'}));

Expected Output

Error with message "nbf" should be a number of seconds for each sign call.

Actual Output

eyJhbGciOiJub25lIiwidHlwIjoiSldUIn0.eyJuYmYiOm51bGwsImlhdCI6MTUzMDAzMTcxM30.
eyJhbGciOiJub25lIiwidHlwIjoiSldUIn0.eyJuYmYiOm51bGwsImlhdCI6MTUzMDAzMTcxM30.
eyJhbGciOiJub25lIiwidHlwIjoiSldUIn0.eyJuYmYiOm51bGwsImlhdCI6MTUzMDAzMTcxM30.

Which decodes as:

{
  "nbf": null,
  "iat": 1530031713
}

[andrewnester]

Hi! I just created PR fixing this: #532