util: removing redundant checks in is* functions
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 <838f779c91673db29b8572ea3f54d191d3878ae2@gmail.com>
Reviewed-By: Michaël Zasso <dc852f7f42c586356f6be32b8780273088b3bded@gmail.com>
Reviewed-By: Jeremiah Senkpiel <f1782d15e31ad502ea2c097c21cc235811d0cb7c@rocketmail.com>

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