Unidirected hyper-edges not working as documented

Reference from [janusgraph-users](https://groups.google.com/d/msg/janusgraph-users/tJnkhLSyMvE/w1lzSnhIAQAJ). Related [documentation](
http://docs.janusgraph.org/latest/advanced-schema.html#_unidirected_edges).

It seems like this functionality worked in older Titan releases, but has not been working since Titan 0.9.0-M2. There must not be any JUnit test cases covering this either.

Start with the linked advanced schema documentation on unidirected edges and compare its behavior with the older Titan releases referenced in the issue. Investigate why the documented functionality stopped working after Titan 0.9.0-M2 and add JUnit coverage for the expected behavior. Done means unidirected hyper-edges work as documented and the regression is covered by tests.

Written by the indexing model from the issue text.