getting a 400 for twitter.com

Seems like twitter.com does not like the request for checking references. Not sure if this is a user agent issue, or just a problem on the twitter.com end.

A small snippet below used for testing:
```python
import pdfx

print(pdfx.downloader.get_status_code('google.com'))
print(pdfx.downloader.get_status_code('twitter.com'))
```