canvas.toDataURL is missing SVG support

1. os debian9
1. node v11.11.0
1. jsbarcode@3.11.0
1. canvas@2.6.1

code:

```javascript
import { createCanvas } from 'canvas';
import jsbarcode from 'jsbarcode';

const canvas = createCanvas(1000, 1000, 'svg');
jsbarcode(canvas, 'Hello');
const barcode_img = canvas.toBuffer() // successfully
// const barcode_img = canvas.toDataURL(); // fails
return {
data: barcode_img,
content_type: 'image/png'
};
```

`toBuffer()` returns successfully, but `toDataURL()` fails. This may be a bug