get_shortest_path doesn't work correctly on undirected graphs

Hi!

I found an example, where `get_shortest_path` didn't works, where it looks like it should:

```elixir
g = Graph.new(type: :undirected) |> Graph.add_vertices([1, 2, 3]) |> Graph.add_edge(1, 3) |> Graph.add_edge(3, 2)
Graph.get_shortest_path(g, 1, 2) # => nil
```

I think, that `get_shortest_path` should raise on undirected graphs or calculate path correctly.