util: removing redundant checks in is* functions

When Object.prototype.toString is used to determine the type, we don't
have to explicitly check for other types. This patch removes the
redundant checks like that.

PR-URL: https://github.com/nodejs/io.js/pull/2179



Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michaël Zasso <mic.besace@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>