Throw correct errors for invalid keys

[BackEndTea]

Tests are based on the specificaitons.

[BenMorel]

Excellent, thank you!

Talking about specifications, it would be nice to be able to run the full test suite from php-src. I actually tried to parse the .phpt files and make a test class that runs them; but it failed because tests are full of var_dump(), which expose the internals of the polyfill and therefore don't look like the native WeakMap.

I guess that makes running them "as is" impossible, however I'd love if you could manually review all weakmap_*.phpt files, and ensure that every test out there is somehow covered in the polyfill's test suite!

[BenMorel]

Released in 0.2.0.