aozora-bunko: Replace invalid and undef in Book#text encoding (#251)

GitHub fixes GH-250

Fix the following error:

```
<Encoding::UndefinedConversionError: "\xF8E" from Shift_JIS to UTF-8>
<Encoding::InvalidByteSequenceError: "\xE7" followed by "\f" on Shift_JIS>
```

Author: abetomo

lib/datasets/aozora-bunko.rb

@@ -94,7 +94,10 @@ def text
         downloader.download(text_file_output_path)
 
         @text = ZipExtractor.new(text_file_output_path).extract_first_file do |input|
-          input.read.encode(Encoding::UTF_8, normalize_encoding(text_file_character_encoding))
+          input.read.encode(Encoding::UTF_8,
+                            normalize_encoding(text_file_character_encoding),
+                            invalid: :replace,
+                            undef: :replace)
         end
 
         @text