CVE 2021 25943 fix

[wid]

Hi,

The CVE-2021-25943 security is now 1 year old and raises npm audit issues.

So, I applied the following recommandation:

#159 (comment)

with units tests into this pull request.

Nothing else has been changed. The npm test is still 100%:
`

[email protected] test
lab -c -l -t 100 -a code
..................................................
..................................................
..................................................
...............
165 tests complete
Test duration: 58 ms
Assertions count: 608 (verbosity: 3.68)
Coverage: 100.00%
Can you please accept this PR and publish an npm package accordingly ?

[wid]

Please note that I throw an error when trying to set forbidden keys.
This is an explicit way of telling the downstream user should really be sure of overriding forbidden keys.