createObjectURL no function was found that matched signature provided

Hi guys,

As the title said I'm trying to use the function saveAs to download files from urls. For the sake of clarity I like to start coding by testable and working examples, so I've used the one on the README.md.

Angular version: 6
My function looks like this:

```
public downloadFile(file: Filesmodel): void {
saveAs('https://httpbin.org/image', 'image.jpg');
}
```

The logs:
```
FileDialogComponent.html:52 ERROR TypeError: Failed to execute 'createObjectURL' on 'URL': No function was found that matched the signature provided.
at new FileSaver (FileSaver.js:127)
at saveAs (FileSaver.js:143)
at FilesService../src/app/services/files.service.ts.FilesService.downloadFile (files.service.ts:47)
at FileDialogComponent../src/app/components/patients/upload-files/file-dialog/file-dialog.component.ts.FileDialogComponent.downloadFile (file-dialog.component.ts:42)
at Object.eval [as handleEvent] (FileDialogComponent.html:53)
at handleEvent (core.js:10259)
at callWithDebugContext (core.js:11352)
at Object.debugHandleEvent [as handleEvent] (core.js:11055)
at dispatchEvent (core.js:7718)
at core.js:8162
```