Fix undirected edges

[gdalle]

fixes #87

@kenahoo can you take it for a spin?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.09%. Comparing base (64990ba) to head (910c09a).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #88      +/-   ##
==========================================
+ Coverage   89.78%   90.09%   +0.30%     
==========================================
  Files           6        6              
  Lines         323      323              
==========================================
+ Hits          290      291       +1     
+ Misses         33       32       -1     

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[kenahoo]

Thanks for the quick fix @gdalle, I tried it out and this works great!