RDoc formatter hardcoded to darkfish for RubyGems

It seems that RDoc is not currently designed to allow a custom formatter when running gem rdoc. Even when using .gemrc and setting the --format option to a custom formatter (like horo) seems to not work. In fact, in the latest version of RDoc (6.0.4), any attempt to use a custom formatter via .gemrc causes a crash:

$ gem rdoc --rdoc --overwrite rake
Parsing documentation for rake-12.3.1
Installing ri documentation for rake-12.3.1
ERROR:  While executing gem ... (TypeError)
    no implicit conversion of nil into String

My .gemrc file:

rdoc: -f horo

I traced the TypeError exception to here: https://github.com/ruby/rdoc/blob/master/lib/rdoc/generator/darkfish.rb#L167

Thanks in advance.

Comece com a falha relatada na linha 167 de lib/rdoc/generator/darkfish.rb e reproduza-a executando gem rdoc --rdoc --overwrite rake com .gemrc configurado como rdoc: -f horo. Rastreie como a opção de formatter personalizado chega ao RDoc e, em seguida, verifique se gem rdoc aceita o formatter sem o TypeError relatado e usa o formatter solicitado.