[Download] Fix URL property issue

The property 'url' of DownloadRequest should not be null, that means setting to null should be ignored, for example:

```
downloadRequest = new tizen.DownloadRequest(url);
downloadRequest.url = null; // downloadRequest.url will not be changed by this statement.
```

Bug: [XWALK-599](https://crosswalk-project.org/jira/browse/XWALK-599)