Handle a case where document has no field 'product'. (Whoosh 2.5 compatibility)

With Whoosh 2.4 all documents have a value in field 'product'
(our workaround to keep faceting from braking). In Whoosh 2.5 this is no longer
the case so this test failed.

Refs: #741


git-svn-id: https://svn.apache.org/repos/asf/bloodhound/trunk@1559082 13f79535-47bb-0310-9956-ffa450edef68