Refine the insync? method for array properties

Make sure to use the sort method only when both `is` and `should` are
arrays. This should prevent errors like "undefined method `sort' for
:absent:Symbol" when a property has never been set for a host,
servicecheck, hosttemplate, etc.
Merge pull request #25 from antaflos/robustify_insync

Refine the insync? method for array properties