Added tests for ko.isSubscribable(null) and ko.isSubscribable(undefined)
Modified ko.isSubscribable to return false for null or undefined instead of crashing
fixed typo in addOrRemoveItem
Merge pull request #863 from zerko/master

Missing ko. in addOrRemoveItem
Merge pull request #861 from bman654/isSubscribable-does-not-crash-on-null-or-undefined

ko.isSubscribable(null) and ko.isSubscribable(undefined) causes an error