Fix centering ascii art

[Lynesth]

Centering ascii art where some lines have trailing spaces are all messed up. This fixes it.
Example of what doesn't work before the fix (select with mouse to see trailing white space):

$art = <<<ART
        _ __ _        
       / |..| \       
       \/ || \/       
        |_''_|        
      PHP SCHOOL      
LEARNING FOR ELEPHANTS
ART;

// What centering actually outputs in a 76 columns width menu:
                               _ __ _                       
                               / |..| \                       
                               \/ || \/                       
                               |_''_|                       
                              PHP SCHOOL                        
                           LEARNING FOR ELEPHANTS                           

Doesn't affect anything else.

[codecov-io]

Codecov Report

Merging #102 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master     #102      +/-   ##
============================================
- Coverage     96.68%   96.68%   -0.01%     
  Complexity      293      293              
============================================
  Files            22       22              
  Lines           905      904       -1     
============================================
- Hits            875      874       -1     
  Misses           30       30

Impacted Files	Coverage Δ	Complexity Δ
src/MenuItem/AsciiArtItem.php	100% <100%> (ø)	14 <0> (ø)	⬇️

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[AydinHassan]

Thanks! could you maybe add the example you gave as a unit test ?

[Lynesth]

Done !