Make url optional

[DzmVasileusky]

We are using ng2-file-upload with a custom upload handler like this:

this.docUploader.onAfterAddingFile = this.handleFileUpload.bind(this);
this.docUploader.onWhenAddingFileFailed = this.handleFileUploadErrors.bind(this);

private handleFileUpload (fileItem: FileItem) { ... convert to Base64 and add to the form model }

So we don't need an automatic upload to an url which is required in version 4 and was optional in version 1.

There is a simple PR which I hope will be merged finally
#1239