Updated _.isFinite to match ES5 and ES6 spec

[TalAter]

The EcmaScript 5 and 6 specs define isFinite as returning false only if ToNumber(object) is NaN, Infinity or -Infinity (a very short and clear read: http://es5.github.com/#x15.1.2.5)

The current implementation of _.isFinite acts as if ToNumber('42') === NaN, and returns false on _.isFinite('42').

This was originally done to combat an "issue" with EcmaScript's ToNumber, which returns true for empty strings (or strings full of nothing but spaces). The problem is that the fix was overzealous, and negates all argument coercion - which is the very heart of isFinite - and acts exactly the opposite of the spec and of the browsers' implementation.

This commit fixes it, while also catching the special gotcha in the spec for empty strings.

// before :
isFinite('12') !== _.isFinite('12') // true !== false
// now:
isFinite('12') === _.isFinite('12') // true === true
// didn't change:
isFinite('')   !== _.isFinite('')   // true !== false

It's also much faster than the current implementation.