Docs-related fix for issue #263

When creating a new issue, passing in a string of comma separated values does not work (no labels are added to the issue). If an array is passed, the labels are then correctly assigned. I'm guessing this is the same for all the other methods which take a `:labels` option? Even in the official GitHub API docs for [creating issues](http://developer.github.com/v3/issues/#create-an-issue) it says that the optional labels input must be an array of strings.