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.

从 lib/rdoc/generator/darkfish.rb 第 167 行报告的失败开始，通过在将 .gemrc 配置为 rdoc: -f horo 的情况下运行 gem rdoc --rdoc --overwrite rake 来复现该失败。跟踪自定义 formatter 选项如何传递到 RDoc，然后验证 gem rdoc 能够接受该 formatter，而不会出现报告的 TypeError，并使用所请求的 formatter。