a little optimized `isEqual` method

https://developer.apple.com/library/mac/documentation/Cocoa/Conceptual/CocoaFundamentals/CocoaObjects/CocoaObjects.html#//apple_ref/doc/uid/TP40002974-CH4-SW61
http://stackoverflow.com/questions/254281/best-practices-for-overriding-isequal-and-hash/254380