cp: Fix guessing if target url is a directory

[vadmeste]

This fixes mc cp /etc/issue/ /tmp/new_dir/ where
/tmp/new_dir/ doesn't exist yet.

[codecov-io]

Codecov Report

Merging #2062 into master will decrease coverage by -0.01%.
The diff coverage is 0%.

@@            Coverage Diff            @@
##           master   #2062      +/-   ##
=========================================
- Coverage    8.38%   8.38%   -0.01%     
=========================================
  Files          90      90              
  Lines        6830    6831       +1     
=========================================
  Hits          573     573              
- Misses       6120    6121       +1     
  Partials      137     137

Impacted Files	Coverage Δ
cmd/common-methods.go	0% <0%> (ø)	✅
cmd/cp-url.go	0% <0%> (ø)	✅

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