Fix the descriptor checks of the Proxy getOwnPropertyDescriptor trap

[saghul]

Fixes #1629.

[bnoordhuis]

Has conflicts. General note: I keep wondering with all these obscure edge case bug reports why test262 doesn't have coverage.

My preference is for people to submit a regression test upstream first. That way we can be sure it's a) an actual corner case and not some misreading of the spec, and b) whether other engines implement it correctly or not.

If not all do, that's probably an indication of something.

[saghul]

FWIW, I submitted a PR with a bunch of extra coverage for iterators and it got approved reasonably quickly, but it's still pending merge :-/