Fix cylinder extent handling

[rdeits]

See rdeits/MeshCat.jl#81

[codecov-io]

Codecov Report

Merging #10 into master will increase coverage by 8.38%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #10      +/-   ##
==========================================
+ Coverage   79.64%   88.02%   +8.38%     
==========================================
  Files           3        3              
  Lines         167      167              
==========================================
+ Hits          133      147      +14     
+ Misses         34       20      -14

Impacted Files	Coverage Δ
src/urdf.jl	82.85% <100%> (+3.8%)	⬆️
src/skeleton.jl	98.3% <0%> (+16.94%)	⬆️

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