Fixed compatibility error with setup command and RDoc plugin on RubyGems

[hsbt]

@mterada1228 @kou After merging ruby/rdoc#1171, RubyGems tests are broken.

TestGemCommandsSetupCommand#test_execute_regenerate_plugins_creates_plugins_dir_if_not_there:
ArgumentError: wrong number of arguments (given 3, expected 1)
    /Users/hsbt/Documents/github.com/ruby/ruby/lib/rdoc/rubygems_hook.rb:277:in 'initialize'
    /Users/hsbt/Documents/github.com/ruby/ruby/lib/rubygems/commands/setup_command.rb:338:in 'Class#new'
    /Users/hsbt/Documents/github.com/ruby/ruby/lib/rubygems/commands/setup_command.rb:338:in 'Gem::Commands::SetupCommand#install_rdoc'
    /Users/hsbt/Documents/github.com/ruby/ruby/lib/rubygems/commands/setup_command.rb:178:in 'Gem::Commands::SetupCommand#execute'
    /Users/hsbt/Documents/github.com/ruby/ruby/test/rubygems/test_gem_commands_setup_command.rb:113:in 'TestGemCommandsSetupCommand#test_execute_regenerate_plugins_creates_plugins_dir_if_not_there'

First, setup_command calls Gem::RDoc.generate directlry in that methods. I expand to argument of RubygemsHook.new and added RubygemsHook#generate for that.

Second, test/rubygems/test_gem_rdoc.rb also uses Gem::RDoc. I added select condition of Gem::RDoc and RDoc::RubyGemshook for that.

[hsbt]

@mterada1228 @kou I'm not sure this is best way to fix with the current issue. Could you also confirm this? After that, I'll pick this change to ruby/rdoc.

[kou]

+1

Sorry for missing this case.

[hsbt]

Thank you for confirming this.