Add bindVars to allowed keys for cursor path

I added bindVars to ALLOWED_KEYS_FOR_PATH so that Query#prepare_request
method does not end up stripping out the option.
This means I can now do this:

````
db = Ashikawa::Core::Database.new(){|conf| conf.url="http://127.0.0.1:8529"}
db.query.execute('FOR t IN TRAVERSAL(imdb_vertices, imdb_edges, "imdb_vertices/759", "outbound", {maxDepth: 2}) FILTER t.vertex.genre == @foo RETURN t', bindVars: {"foo" => "Comedy"}).to_a
````