Use SPECIAL_PATH_SEGMENT when encoding path in from_file_path

[valenting]

Fixes #901

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (92f356e) 81.70% compared to head (1ce0c83) 81.75%.
Report is 1 commits behind head on master.

Files	Patch %	Lines
idna/tests/punycode.rs	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #902      +/-   ##
==========================================
+ Coverage   81.70%   81.75%   +0.04%     
==========================================
  Files          20       20              
  Lines        3547     3551       +4     
==========================================
+ Hits         2898     2903       +5     
+ Misses        649      648       -1     

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[lucacasonato]

LGTM

[edgul]

Looks good!