Do not put binary illustration data into a string.

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

Fix #73

lib/src/main/cpp/libkiwix/illustration.cpp

@@ -50,4 +50,7 @@ METHOD0(jstring, url) {
   return TO_JNI(THIS->url);
 } CATCH_EXCEPTION(nullptr)
 
-GETTER(jstring, getData)
+METHOD0(jbyteArray, getData) {
+  auto data = THIS->getData();
+  return cArray2jni(data->c_str(), data->size(), env)
+} CATCH_EXCEPTION(nullptr)

lib/src/main/java/org/kiwix/libkiwix/Illustration.java

@@ -8,7 +8,7 @@ public class Illustration
   public native String mimeType();
   public native String url();
 
-  public native String getData();
+  public native byte[] getData();
   @Override
   protected void finalize() { dispose();  }