assert.deepEqual: fix bug with faked boxed primitives

I discovered this bug while perusing the code, to try to bring the npm deep-equal package into sync with node's.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
  • note: the related tests have passed; i didn't run the full suite
• tests and/or benchmarks are included
• documentation is changed or added (n/a)
• commit message follows commit guidelines