Added test where addToSet strategy on embedMany results in nested arrays...

I believe this is the same problem as in issue #665, where @jmikola requested a test case showing this.

In the following test, the embedded collection "embedded" behaves as it should. "embeddedset" does not.

I branched it from 1.0.0-BETA9 because almost all the tests run successfully there for me, which is not the case at the master branch head. I hope that's not a problem.