ostruct.rb: fix OpenStruct.allocate

	* lib/ostruct.rb (OpenStruct.allocate): initialize an instance
	  variable directly, without calling `intialize` method which may
	  be overridden in a subclass.  [ruby-core:80292] [Bug #13358]
	ostruct.rb: improve fix for OpenStruct.allocate + #respond_to?

	* lib/ostruct.rb (OpenStruct#respond_to_missing?): this makes
	  OpenStruct#respond_to? works on any OpenStruct instance,
	  just like Kernel#respond_to? does, without workarounds.
	  [ruby-core:80292] [Bug #13358]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_4@59407 b2dd03c8-39d4-4d8f-98ff-823fe69b080e