Require RDoc in input-method.rb again in a limited scope.

[junaruga]

This PR is a follow up by #393 (comment).

RDoc is implemented as soft dependency in IRB. See how the rdoc is required in the files. I reverted the commit below.

$ grep -ril rdoc lib/
lib/irb/cmd/help.rb
lib/irb/completion.rb
lib/irb/easter-egg.rb
lib/irb/input-method.rb

Ideally I wanted to run test the case of 'no rdoc' with the logic in test/lib/helper.rb below. But I couldn't do it. Perhaps the Kernel#require was overridden by Rubygems or Bundler in the process of running unit-test.

if ENV["NO_RDOC"] == "true"
  module Kernel
    alias_method :old_require, :require
    def require(name)
      raise LoadError, "cannot load such file -- rdoc (test)" if name == "rdoc"
      old_require(name)
    end
  end
end

require 'irb'

---

Revert "Remove require in signal handler to avoid ThreadError"

This reverts commit 5f749c6.

[hsbt]

👍 Nice catch, Thanks.