bug: extend function assign array instead of cloning

[tvanbeek]

Hey me again, found another one :)

On line 142 of ModelMap.js it says:

var labels = original.labels();

But this makes a reference to the array on the original model instead of cloning it. Therefor if when you inherit (or multiple inherit) schema's then all the models will hold the same labels.

To fix it simply rewrite to:

var labels = original.labels().slice(0);

So, combined with the other bug on line 143 the whole function could look like this:

    // starting from line 136 in ModelMap.js
    key: 'extend',
            value: function extend(name, as, using) {
            // Get Original Model
            var original = this.models.get(name);

            // Add new Labels
            var labels = original.labels().slice(0);
            labels.push(as);
            labels.sort();

            // Merge Schema
            var schema = Object.assign({}, original.schema(), using);

            // Create and set
            var model = new _Model2.default(this._neode, as, schema);

            model.setLabels.apply(model, _toConsumableArray(labels));

            this.models.set(as, model);

            return model;
        }