clippingnode.js _initForCanvas don't return true

[Chleba]

When rendering mode is set to canvas, clipping nodes is not having init method.
Method _initForCanvas need's to have - return true

_initForCanvas: function (stencil) {
    this._stencil = stencil;
    this.alphaThreshold = 1;
    this.inverted = false;
    return true;
},

[dingpinglv]

Thanks for notice, @Chleba .