Floyd-Warshall a corner case

Thanks for creating graphlib and dagre!  We're thinking of using them at Concurix.

While experimenting with graphlib, I found and hopefully fixed, a corner case for Floyd-Warshall.  I think it should take the minimum weight among all self edges as the shortest path from a node to itself, with a maximum of 0.  The original code would pick the weight of one of the incident edges, not necessarily the minimum weight and sometimes  a positive weight.