Fix multi-byte characters sanitization in short name field

[thelovekesh]

Fixes: #971

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (456f039) 19.65% compared to head (82e77da) 19.65%.

Additional details and impacted files

@@            Coverage Diff             @@
##             develop     #984   +/-   ##
==========================================
  Coverage      19.65%   19.65%           
  Complexity       347      347           
==========================================
  Files             57       57           
  Lines           2325     2325           
==========================================
  Hits             457      457           
  Misses          1868     1868           

Flag	Coverage Δ
php	19.65% <100.00%> (ø)
unit	19.65% <100.00%> (ø)

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Impacted Files	Coverage Δ
wp-includes/class-wp-web-app-manifest.php	66.52% <100.00%> (ø)

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[westonruter]

Thanks!