Ruby

forwardable.rb: use defined?

• lib/forwardable.rb (_delegator_method): use defined? operator
  instead of binding and calling unbound Kernel#respond_to?. a
  remedy for an performance bottleneck.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@57256 b2dd03c8-39d4-4d8f-98ff-823fe69b080e