path: fix win32.isAbsolute() inconsistency

This commit fixes an inconsistency in absolute path checking compared
to the absolute path detection used by the other path.win32 functions.

Fixes: https://github.com/nodejs/node/issues/6027
PR-URL: https://github.com/nodejs/node/pull/6028


Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>