Bug when processor defines both datasource-independent and datasource-specific properties

It seems we never ran into this, but in the course of #3547199: Allow the "Custom value" token property to be added to a specific datasource I noticed that if a processor defines both a datasource-specific and a datasource-independent property, and both are present on a specific item, \Drupal\search_api\Item\Item::getFields() will actually invoke $processor->addFieldValues() for that processor, potentially leading to duplicate field values.

The fix is simple enough but will also need a regression test.