fix for Floyd-Warshall algorithm for non-default weighted edges 

hello,

Algorithm has incorrectly assumed that all edges are weighted with 1.0. That is not always true, so I fixed this in code. I've also written a new test to show that this works. 
My change does not fail any existing UnitTests.

please apply this fix to main code branch.

thank you for excellent job you're doing with jgrapht library!
yours,
Mike