Support multiple datasets in @ExpectedDatabase

[thombergs]

I'm trying to use spring-test-dbunit to create minimal datasets for integration tests. To achieve this goal, it would be great to be able to put multiple additive datasets into the @ExpectedDatabase annotation just like into the @DatabaseSetup annotation.

[thombergs]

It'd be awesome if this PR would make it into the next release, since then it would be possible to create really modular test database files!