Parsing documentation for WinRM fails

[Ana06]

With Ruby 2.6, rdoc 6.1.0 and last version of WinRM

> ruby  -c lib/winrm/psrp/powershell_output_decoder.rb
Syntax OK

> sudo gem install winrm
Parsing documentation for winrm-2.3.1
Before reporting this, could you check that the file you're documenting
has proper syntax:

  /usr/bin/ruby.ruby2.6 -c lib/winrm/psrp/powershell_output_decoder.rb

RDoc is not a full Ruby parser and will fail when fed invalid ruby programs.

The internal error was:

        (NoMethodError) undefined method `[]' for nil:NilClass

ERROR:  While executing gem ... (NoMethodError)
    undefined method `[]' for nil:NilClass
        /usr/lib64/ruby/2.6.0/rdoc/parser/ripper_state_lex.rb:496:in `heredoc_end?'
        /usr/lib64/ruby/2.6.0/rdoc/parser/ripper_state_lex.rb:473:in `get_heredoc_tk'
        /usr/lib64/ruby/2.6.0/rdoc/parser/ripper_state_lex.rb:346:in `get_squashed_tk'
        /usr/lib64/ruby/2.6.0/rdoc/parser/ripper_state_lex.rb:578:in `parse'
        /usr/lib64/ruby/2.6.0/rdoc/parser/ruby.rb:180:in `initialize'
        /usr/lib64/ruby/2.6.0/rdoc/parser.rb:191:in `new'
        /usr/lib64/ruby/2.6.0/rdoc/parser.rb:191:in `for'
        /usr/lib64/ruby/2.6.0/rdoc/rdoc.rb:360:in `parse_file'
        /usr/lib64/ruby/2.6.0/rdoc/rdoc.rb:416:in `block in parse_files'
        /usr/lib64/ruby/2.6.0/rdoc/rdoc.rb:414:in `map'
        /usr/lib64/ruby/2.6.0/rdoc/rdoc.rb:414:in `parse_files'
        /usr/lib64/ruby/2.6.0/rdoc/rubygems_hook.rb:190:in `block in generate'
        /usr/lib64/ruby/2.6.0/rdoc/rubygems_hook.rb:189:in `chdir'
        /usr/lib64/ruby/2.6.0/rdoc/rubygems_hook.rb:189:in `generate'
        /usr/lib64/ruby/2.6.0/rdoc/rubygems_hook.rb:56:in `block in generation_hook'
        /usr/lib64/ruby/2.6.0/rdoc/rubygems_hook.rb:55:in `each'
        /usr/lib64/ruby/2.6.0/rdoc/rubygems_hook.rb:55:in `generation_hook'
        /usr/lib64/ruby/2.6.0/rubygems/request_set.rb:323:in `block in install_hooks'
        /usr/lib64/ruby/2.6.0/rubygems/request_set.rb:322:in `each'
        /usr/lib64/ruby/2.6.0/rubygems/request_set.rb:322:in `install_hooks'
        /usr/lib64/ruby/2.6.0/rubygems/request_set.rb:221:in `install'
        /usr/lib64/ruby/2.6.0/rubygems/commands/install_command.rb:212:in `install_gem'
        /usr/lib64/ruby/2.6.0/rubygems/commands/install_command.rb:264:in `block in install_gems'
        /usr/lib64/ruby/2.6.0/rubygems/commands/install_command.rb:258:in `each'
        /usr/lib64/ruby/2.6.0/rubygems/commands/install_command.rb:258:in `install_gems'
        /usr/lib64/ruby/2.6.0/rubygems/commands/install_command.rb:165:in `execute'
        /usr/lib64/ruby/2.6.0/rubygems/command.rb:321:in `invoke_with_build_args'
        /usr/lib64/ruby/2.6.0/rubygems/command_manager.rb:176:in `process_args'
        /usr/lib64/ruby/2.6.0/rubygems/command_manager.rb:146:in `run'
        /usr/lib64/ruby/2.6.0/rubygems/gem_runner.rb:59:in `run'
        /usr/bin/gem:21:in `<main>'

> sudo gem install winrm --no-document
Successfully installed winrm-2.3.1
1 gem installed

It seems related to #697 and #694