test_keyword.rb: check error messages

* test/ruby/test_keyword.rb (test_required_keyword): check also
  error messages.

* test/ruby/test_keyword.rb (test_block_required_keyword): ditto.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44067 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
class.c: optimization just one key

* class.c (keyword_error): use only element itself for
  optimization in the case the key has just one element.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44068 b2dd03c8-39d4-4d8f-98ff-823fe69b080e