Support String version field in Integration test

gvNIX occ-checksum add-on allows to use Optimistic Concurrency Control without add a "version" persistent field to a entity.

This feature is really useful on project which has to use RE on legacy database and must left unchanged database schema.

This requires to add a String transient field to entity to hold computed checksum based on record contents.

When integration test is added to entities managed by this add-on some test (flushTest and mergeTest) has compilation errors because it uses the ">" comparator to check version field.

We need support version field of type String in integration fields.

I'll prepare a pull-request which solve this problem.