extension.rdoc: fix rb_get_kwargs [ci skip]

	* doc/extension.rdoc: [DOC] optional keyword arguments are
  defaulted to Qundef.  ignored keys are kept in the hash but a
  new Hash is not created.  [ruby-dev:49893] [Bug #13004]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_3@57346 b2dd03c8-39d4-4d8f-98ff-823fe69b080e