CanvasRenderingContext2D class name

[arusakov]

Rendering context called "CanvasRenderingContext2D" in the [specification].(https://html.spec.whatwg.org/#2dcontext)
Also I saw [object CanvasRenderingContext2D] in Chrome, FF and Safari after this code:

document.createElement('canvas').getContext('2d').toString();

[LinusU]

Thanks 🙌

(merging as soon as Travis is done)