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!
I see where you're coming from, and initially, I thought the same, but technically, the ZF2 DB layer is not scrambling anything; rather, it is abiding by the column-casing set by the value of the driver option db2_attr_case, which can be one of DB2_CASE_LOWER, DB2_CASE_UPPER, or DB2_CASE_NATURAL.  The default is DB2_CASE_UPPER, and is used by nearly every IBM shop we at Zend have worked with.

Since CredentialTreatmentAdapter::authenticateValidateResult() is always looking for a lowercase key, the value of db2_attr_case might or might not allow authenticate() to behave as expected, depending on the setting of db2_attr_case for a given application.

Should this really go somewhere in the DB layer, or should this go in an IBM-specific class overriding CredentialTreatmentAdapter::authenticateQuerySelect(), or should it stay where it is?