Support trailing slashes on urls for coalesceFindMany.

Currently if you override your adapter as follows, coalesceFindRequests will break (silently).  
```javascript
export default DS.ActiveModelAdapter.extend({
    buildURL: function(type, id, record){
      return this._super(type, id, record) + '/';
    },
});
```
As _stripIDFromURL is marked as a private method I do not think it should be expected that users override it.  This provides a simple fix.  Happy to add a test if someone can suggest a simple approach for testing this.