syncer calls onChangeHandler with wrong arguments with existing data

[nilclass]

In line 420+, syncer/include.js calls the onChangeHandler for all items in all collections.
It gets called with wrong arguments:

• "key" is the index in the array of items instead of the actual key
• "newValue" is null, as getItem is called with the invalid key as well.

This is due to getCollection, which doesn't return any keys. Michiel mentioned in the channel, that in the original usecase this wasn't needed, as the items had their key duplicated as an "id" attribute. Depending on this may or may not be a good idea.
Either way, the bug described above is still a bug.

[nilclass]

I solved this in 4cdab2c, by allowing getCollection to be called in two ways.

This is how it was (and still works):

syncer.getCollection('contacts') //=> [{...}, {...}, {...}, ...]

This is the new way:

syncer.getCollection('contacts', function(contact, key) {
   // ...
}); //=> null