Collection#set treats Object.prototypes as existing Models

[metafeather]

Ran into this as a bug in Firefox only due to my data which errors as:

TypeError: existing.set is not a function

this is because one of my model.id is 'watch' (for a wrist watch) however Backbone is checking this id in the Collections._byId cache and matching theObject.watch() method.

A reduced example is here: http://jsbin.com/akaluh/1/edit

The console output for Firefox is [native code] whereas Chrome and Safari it is undefined.

This is similar to other bugs/issues such as: #964, #1495, #2296.

I've introduced a check that the return from Collection.get is actually an Object - I can't think why it would ever need to be something else? Pull request coming shortly.

I also investigated using this._byId.hasOwnProperty() however it broke my app. I think that because Collections can be extended the _byId object may not be on 'this' collection.