Restore the `call` and `apply` methods of Function when adding a chainable method.

This was broken on platforms that have `__proto__` support.
Details at https://github.com/chaijs/chai/commit/514dd6ce4#commitcomment-2593383
Merge pull request #140 from RubenVerborgh/function-prototype

Restore the `call` and `apply` methods when adding a chainable method