FIX: issue #76 (Querying indexed field in subdocument does not work, but works without index)

I've added a test, pointing out the problem referred in issue #76 and fixed it ;-)

The root cause seems to be within class IndexAbstract.
When an index was created, it couldn't figure out,
if a BasicDBList within a DBObject contains a field or not.
IMHO if there are arrays within a DBObject, this object should go into the index, to be queried later.

Feedback is welcome.

Regards
Martin