Updated indexer for book url

[achantz]

Updated array indexer to include charset=utf-8 in JSON output. This was returning an empty string and would skip the fetching of the book text, since it could not determine the actual book url.

Sample JSON output from page fetch

"formats": {
        "text/html": "https://www.gutenberg.org/ebooks/768.html.images",
        "application/epub+zip": "https://www.gutenberg.org/ebooks/768.epub3.images",
        "application/x-mobipocket-ebook": "https://www.gutenberg.org/ebooks/768.kf8.images",
        "application/rdf+xml": "https://www.gutenberg.org/ebooks/768.rdf",
        "image/jpeg": "https://www.gutenberg.org/cache/epub/768/pg768.cover.medium.jpg",
        "application/octet-stream": "https://www.gutenberg.org/cache/epub/768/pg768-h.zip",
        HERE --> "text/plain; charset=utf-8": "https://www.gutenberg.org/ebooks/768.txt.utf-8"
      },