Clear the charset setting by setting it to nil in ruby-land if the charset is detected to be 'none'.
If the user happens to have set the value to 'none' in a config file, or it's otherwise specified, it appears the underlying charset detection mechanism in the C++ API treats it as an encoding. So, by setting it to 'nil' directly in the ruby code, we'll make the correct call to SetCharset(0) as the user probably intends.