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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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](https://app.codecov.io/gh/kiwix/java-libkiwix/pull/74?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=kiwix) | Coverage Δ | | |---|---|---| | [...src/main/java/org/kiwix/libkiwix/Illustration.java](https://app.codecov.io/gh/kiwix/java-libkiwix/pull/74?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=kiwix#diff-bGliL3NyYy9tYWluL2phdmEvb3JnL2tpd2l4L2xpYmtpd2l4L0lsbHVzdHJhdGlvbi5qYXZh) | `100.00% <ø> (ø)` | | | [lib/src/test/org/kiwix/test/libkiwix/TestBook.java](https://app.codecov.io/gh/kiwix/java-libkiwix/pull/74?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=kiwix#diff-bGliL3NyYy90ZXN0L29yZy9raXdpeC90ZXN0L2xpYmtpd2l4L1Rlc3RCb29rLmphdmE=) | `96.15% <100.00%> (ø)` | | | [...test/org/kiwix/test/libkiwix/TestIllustration.java](https://app.codecov.io/gh/kiwix/java-libkiwix/pull/74?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=kiwix#diff-bGliL3NyYy90ZXN0L29yZy9raXdpeC90ZXN0L2xpYmtpd2l4L1Rlc3RJbGx1c3RyYXRpb24uamF2YQ==) | `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).