Support g:T for .NET

We didn't have tests for return of T which can occur with valueMap(true) - adding them on TINKERPOP-1860 exposed the failure in .NET. This may or may not be an issue for GraphSON 3.0 only. In GraphSON 2.0, g:T is a defined type, but it is typically not returned from the server outside of valueMap(true). When that did happen in GraphSON 2.0, T values were simply converted to string.