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 adds specs from Backbone repo (minus noconflict spec, since it was broken), and fixes another bug where passing `{merge: true}` to `collection#add` failed.