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!
 > The default is DB2_CASE_UPPER, and is used by nearly every IBM shop we at Zend have worked with.

That by far means that DB2 in general is doing it right :-)

The fix is simple, so I won't be too picky about it, but you may as well introduce it as `@todo` if `Zend\Db` can't abstract this ugly detail of the RDBMS away from us right now (and by abstracting, I mean reading the `db2_attr_case` and doing something about it).
I think the keys should just match the input queries, and so should the return values.

If you don't plan to fix that in this PR (an additional `if` condition is acceptable over a huge rework of `Zend\Db`), then please open an issue about it and reference it in a `@todo`.