Fix 6428 - authenticate() always fails on IBMi when using DB table-based authentication

Here is the issue report which this pull request addresses - https://github.com/zendframework/zf2/issues/6428

One-line addition to the AbstractAdapter, a set of unit tests specific for IBM i / DB2 (run on the IBM, of course, against a DB2 table), and some additional TestConfiguration constants defined for controlling the tests.

Bear with me: this is my first ZF2 pull request!
Perhaps I'm thinking too hard about it, but the more I consider it, the less I see that Zend\Db needs any change. It is doing exactly what it is supposed to do: return column names cased according to db2_attr_case, regardless of the case used when the query was built.

The values returned are whatever was stored in the table; db2_attr_case affects only the column names, and thus the keys of our associative arrays.

I could be totally missing your point, and if so, I apologize, but is your preference special code in Zend\Db that looks for a ZEND_AUTH_CREDENTIAL_MATCH column (either upper or lower case) in a resultset, and forces it lower case, so that the authentication code sees the lowercase array key it is looking for?