Add test for difference, addresses #446 bug.
Add test for difference, addresses #446 bug.

jtorrents/networkx,bzero/networkx,blublud/networkx,kernc/networkx,ghdk/networkx,JamesClough/networkx,dhimmel/networkx,wasade/networkx,aureooms/networkx,farhaanbukhsh/networkx,sharifulgeo/networkx,sharifulgeo/networkx,jcurbelo/networkx,jni/networkx,ionanrozenfeld/networkx,aureooms/networkx,jni/networkx,aureooms/networkx,kai5263499/networkx,sharifulgeo/networkx,dmoliveira/networkx,ionanrozenfeld/networkx,chrisnatali/networkx,kai5263499/networkx,RMKD/networkx,ionanrozenfeld/networkx,bzero/networkx,nathania/networkx,blublud/networkx,andnovar/networkx,cmtm/networkx,michaelpacer/networkx,farhaanbukhsh/networkx,kai5263499/networkx,chrisnatali/networkx,jakevdp/networkx,debsankha/networkx,nathania/networkx,OrkoHunter/networkx,chrisnatali/networkx,jtorrents/networkx,dhimmel/networkx,goulu/networkx,harlowja/networkx,RMKD/networkx,ghdk/networkx,Sixshaman/networkx,farhaanbukhsh/networkx,SanketDG/networkx,NvanAdrichem/networkx,jni/networkx,jakevdp/networkx,dmoliveira/networkx,bzero/networkx,blublud/networkx,kernc/networkx,RMKD/networkx,harlowja/networkx,dhimmel/networkx,debsankha/networkx,beni55/networkx,jfinkels/networkx,yashu-seth/networkx,dmoliveira/networkx,jakevdp/networkx,ltiao/networkx,debsankha/networkx,harlowja/networkx,tmilicic/networkx,nathania/networkx,kernc/networkx,ghdk/networkx