js: revert changes to class read/write restriction
- undoes 94ee0988d4eaac27785de6efb7c19ca9976e1e9c and e16a87e83910ebb6bfdc4148369165f121f0997e. I do not fully understand why my approach was bad, but I am not in a position to argue. This revert was requested to me after a review, and I'm doing it in the assumption that more experienced people know better than me.
- unittests: Also changed some outdated tests that stopped working since 754e799d35fd62d7a979452f44342934659908c7 (as GPGME_Key is not exported, one cannot check for instanceof in the tests anymore)