update is_first and is_last functions

[amiyasahu]

https://github.com/amiyasahu/q2a-social-share/blob/master/qa-social-share-utils.php#L173

references - http://stackoverflow.com/questions/1028668/get-first-key-in-a-possibly-associative-array http://stackoverflow.com/questions/1921421/get-the-first-element-of-an-array http://stackoverflow.com/questions/6251331/get-key-of-the-last-element-in-an-array http://stackoverflow.com/questions/2260912/checking-if-a-key-is-the-last-element-in-an-array