Fixed _.isArray() bug

I'm not sure what changed, but Octokit suddenly stopped working for me today. GitHub was returning a 422 error on a commit and I tracked it down to the parents property in the request. Octokit was sending a string instead of an array, which GitHub doesn't like. It turned out that the _.isArray() function was returning true for a string so the commit() function was not changing it to an array so a string was being sent in the request.

I simply changed the array test to use instanceof instead of testing for the existence of the slice function, which a javascript string has.