Replace undefined chars by empty string

Some chars can't be converted in ASCII because there is no equivalence.

For instance the following character : [`’`](http://www.fileformat.info/info/unicode/char/2019/index.htm)

Setting the `:undef` options ignores them. This prevents the following exception to occur when trying the conversion.

```
Encoding::UndefinedConversionError: U+2019 from UTF-8 to ISO-8859-1
```