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.
	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.
	forwardable.rb: fix branches

	* lib/forwardable.rb (instance_delegate, single_delegate): fix
	  inverted branches.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_4@57913 b2dd03c8-39d4-4d8f-98ff-823fe69b080e