Passing hasMany relationship into {{#sortable-objects}}

[ianpetzer]

Hi there,

I'm struggling to pass a hasMany relationship into a {{#sortable-objects}}. It works fine in the simple case of a single sortable list but I'm finding that if either:

1. Use a single list and set inPlace=true
2. Use multiple lists and drag from one list to a list to another where the sortObjectList is a hasMany relationship I get the following error:

items.findIndex is not a function

This is coming from:

ember-drag-drop/addon/services/drag-coordinator.js

Line 8 in 2f29c0e

return items.findIndex(function (element) {

as a hasMany relationship does not support the findIndex function

[ianpetzer]

This PR #176 solves the issue in my specific case of using an ember hasMany proxy. Would this be usable in all scenarios?