add spec for ENV.except

[lxxxvi]

Hello 👋

Here's my take on ENV.except from #823

ENV.except has been added, which returns a hash excluding the
given keys and their values. Feature #15822

Two notes:

• Creating the spec for ENV.except using ../mspec/bin/mkspec -b core -c ENV didn't work, I created the file manually.
• Testing .except without arguments is covered via it_behaves_like :env_to_hash, :except which is not super-obvious, I guess.

Please letme know if you like any changes etc.

Thanks, as always. 🙌

[eregon]

• Testing .except without arguments is covered via it_behaves_like :env_to_hash, :except which is not super-obvious, I guess.

Could you add a comment about that above the it_behaves_like?

[eregon]

Looks good, thanks!

[lxxxvi]

@eregon Thanks! I added the comment.