fix: Correct multiple author display (#856)

## Summary

* If an EPUB has:
```
<dc:creator>J.R.R. Tolkien</dc:creator>
<dc:creator>Christopher Tolkien</dc:creator>
```
the current result for epub.author would provide : "J.R.R.
TolkienChristopher Tolkien" (no separator!)
* The fix will seperate multiple authors: "J.R.R. Tolkien, Christopher
Tolkien"

## Additional Context

* Simple fix in ContentOpfParser - I am not seeing any dependence on the
wrong concatenated result.

---

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lib/Epub/Epub/parsers/ContentOpfParser.cpp

@@ -327,6 +327,9 @@ void XMLCALL ContentOpfParser::characterData(void* userData, const XML_Char* s,
   }
 
   if (self->state == IN_BOOK_AUTHOR) {
+    if (!self->author.empty()) {
+      self->author.append(", ");  // Add separator for multiple authors
+    }
     self->author.append(s, len);
     return;
   }