vm: fix property descriptors of sandbox properties

The GlobalPropertyQueryCallback was changed in 2010 to return an
integer instead of a boolean:

https://groups.google.com/forum/#!topic/v8-users/OOjHJrix-cU

This integer communicates the property descriptors of the property,
instead of just its presence or absence. However, the original
contextify code was probably written before this change, and it was
not updated when porting to Node.js.

Credit to @smikes for the test and the original PR of #885.

Fixes: https://github.com/nodejs/io.js/pull/864
Fixes: https://github.com/nodejs/io.js/pull/885
PR-URL: https://github.com/nodejs/io.js/pull/1773


Reviewed-By: Fedor Indutny <fedor@indutny.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>