Fix core.slash_split() to strip both leading and trailing slashes

[dmach]

Fixes: #1477

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (cff6a0c) 31.51% compared to head (dec4143) 31.51%.
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1486   +/-   ##
=======================================
  Coverage   31.51%   31.51%           
=======================================
  Files          51       51           
  Lines       17938    17938           
=======================================
  Hits         5653     5653           
  Misses      12285    12285           

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