Fix for setStringType in DataSource

[alexismeneses]

Integration of fix #192 submitted by @einavitamar

In a more general way, I think we should use a class of constants grouping all supported driver properties instead of leaving strings a bit everywhere in the code where this kind of mistake is possible.
Making this change can introduce big diff between master and older branches but with the upcoming release of 9.4, we may have less need to backpatch, thus reducing the impact.
Any advice before I possibly work on that?

[davecramer]

This seems like a worthwhile goal. I'd say go for it and it won't get backpatched into 9.3

[ringerc]

There's a growing diff between 9.3 and 9.4, such that I see the 9.3 driver as the end of a "legacy" driver series, and the 9.4 driver as the beginning of a driver that'll hopefully get progressively cleaned, trimmed down and have cruft removed. I'm not immensely concerned about backpatches.