additional_urls field is not populated with the list of canonical URLs

## Bug Description
After the crawl is performed using opencrawler, we don't seem find any field called additional_urls with the list of canonical URLs. We are very sure that the canonical URL exist on the pages.

### To Reproduce
Steps to reproduce the behavior:
1. Perform crawl
2. Notice the crawled document in the index
3. additional_urls is not found in the crawled document

## Expected behavior
additional_urls should list the canonical URLs in the crawled document

## Screenshots

## Environment

- OS: [e.g. iOS]
- Browser [e.g. chrome, safari]
- Version [e.g. 22]

## Additional context