In Backbone.Collection#_addWithIdCheck check for model(s).
In Backbone 1.0.0, Collection#reset does a Collection#add with no
model(s), which causes this method to fail when it looks for an id
property on the missing model parameter.
Also add backbone specs, provide fix for collction#add {merge:true}
When adding a new model, pass `{merge: true`} options.
Also no need to instantiate model before adding to collection. This has
the added bonus of preventing a shadow collection being created for this
model by Backbone Relational.
Merge pull request #1 from lukeasrodgers/bugfix_idempotent_collection

In Backbone.Collection#_addWithIdCheck, check for model(s).