Don't overwrite PDFJS.workerSrc if already set

[bascoder]

If PDFJS.workerSrc isn't overwritten when it's set already, then client code is able to set it to the preferred URL.

Related to issue #162

For instance, my client code can overwrite PDFJS.workerSrc as following:

export class MyComponent implements OnInit {
  ngOnInit() {
    window.PDFJS.workerSrc = '/pdf.worker.min.js';
  }
}

[pengisgood]

It also affected my project, could you please create another release including this change?