Fix to support IE8

*Problem:*
`Array.isArray` is not supported in IE8. (http://kangax.github.io/es5-compat-table/#Array.isArray)
This is the only thing that prevents this library from being used in IE8

*Solution:*
Replacing the use of that method with `Object.prototype.toString.call(arg) === "[object Array]";` as recommended by MDN (https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/isArray#Compatibility)

Also fixed incorrect indentation.