Update to API-gateway user header

https://github.com/adsabs/solr-service/blob/a1f7909a1406e7820ecb8326ac8728dd1dfbd419/solr/views.py#L218

This line needs to be updated to 

```python
   user_id = request.headers.get('X-api-uid', 'default')
```

It was corrected in the original `doctype_boost` hotfix, but has not been merged into the mainline `solr-service`