Fix EngineVersion for PendingModifiedValues (plus minor cleanup)

Changes handling of 'EngineVersion' in the RDS DB parser so that it will correctly appear in the PendingModifiedValues hash. (Note the differences in Fog's response versus the proper one from Amazon.) Previously, 'EngineVersion' was appearing in two places in the response parser code; the second occurrence would never be processed.

I also moved 'Iops' and 'AllocatedStorage' to be handled by the same code block, to reduce code duplication.