EZP-21765: Fix ezfSolrDocumentFieldDummyExample::getClassAttributeType()

When running the eZ Find Unit Test Suite, an error is thrown, because `ezfSolrDocumentFieldDummyExample::getClassAttributeType()` doesn't match its parent `ezfSolrDocumentFieldDummyExample::ezfSolrDocumentFieldBase()`.

This PR fixes that.

Cheers
:octocat: Jérôme