Allow ISO8859-1 encoding in properties

[cmaglie]

A .properties file is parsed as UTF-8 encoded, but if we found some non UTF-8 characters, we now assume it's ISO8859-1 and we convert it back to UTF-8. It is common to find ISO8859-1 encoded .properties files in old java projects.

ISO8859-1 range is 0x00-0xFF so the conversion is done by simply converting each byte in the corresponding code point in UTF-8: https://stackoverflow.com/a/13511463/1655275

rune is an alias for int32, and when it comes to encoding, a rune is assumed to have a Unicode character value (code point). So the value b in rune(b) should be a unicode value. For 0x00 - 0xFF this value is identical to Latin-1, so you don't have to worry about it.

Then you need to encode the runes into UTF8. But this encoding is simply done by converting a []rune to string.