On unpack, one may want to mix `JSON_STRICT` and optional keys by using
a format like `{s:i,s?o!}`. Unfortunately, this fails the stric test
with `-1 object item(s) left unpacked` error when the second key is not
specified.
To fix that, we iter on each key and we check if we have successfully
unpacked them. This is less efficient than the previous method but it
brings correctness.
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