path: reduce type checking on some methods

a465840313f548b913eb2bd8ea3d26c2ab5dcebb added strict type
checking for the methods in the path module. However, dirname(),
basename(), and extname() actually had some undocumented uses
in the wild. This commit loosens the type checking on those
methods.

Fixes: https://github.com/iojs/io.js/issues/1215
PR-URL: https://github.com/iojs/io.js/pull/1216


Reviewed-By: Rod Vagg <rod@vagg.org>