Steps to reproduce

1. Prepare a test eZ Publish + ezsystems/ezplatform-solr-search-engine + netgen/ezplatformsearch environmewnt, fully updated via composer. Make sure the Solr service is running;

Ref: https://doc.ez.no/display/EZP/Solr+Search+Engine+Bundle

2. On the eZ Publish admin interface, create a test article called "Article 1". It will have a default priority value of "0";
3. On the Solr admin interface, search for the newly created article e.g. http://localhost:8983/solr/collection1/select?q=%22Article+1%22&wt=json&indent=true
4. On the resulting json structure, verify that the priority is as expected:

...
"main_location_priority_i": 0,
...

5. Back to the eZ Publish admin interface, change the article priority e.g. "1";
6. On the Solr admin interface, search again for "Article 1". The priority is still "0", it should now be "1";

Ref: https://github.com/netgen/ezplatformsearch/issues/11