Do not put binary illustration data into a string.

[mgautierfr]

Java String must contains utf8 content.
Illusration data which is PNG is definitively not a utf8 string.

Fix #73

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (853233a) 92.38% compared to head (ed6a302) 92.54%.

Additional details and impacted files

@@             Coverage Diff              @@
##               main      #74      +/-   ##
============================================
+ Coverage     92.38%   92.54%   +0.15%     
- Complexity      213      219       +6     
============================================
  Files            44       45       +1     
  Lines           289      295       +6     
  Branches          3        3              
============================================
+ Hits            267      273       +6     
  Misses           19       19              
  Partials          3        3              

Files	Coverage Δ
...src/main/java/org/kiwix/libkiwix/Illustration.java	100.00% <ø> (ø)
lib/src/test/org/kiwix/test/libkiwix/TestBook.java	96.15% <100.00%> (ø)
...test/org/kiwix/test/libkiwix/TestIllustration.java	100.00% <100.00%> (ø)

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[kelson42]

@mgautierfr Thank you for the quick response!
@MohitMaliDeveloper Test and review. If OK, prepare the release 1.0.1 (see the ticket).

[MohitMaliFtechiz]

@mgautierfr Thanks for your quick fix, now the issue is resolved 🎉 🎉, i am able to get the favicon from the book.

LGTM.