* misc/ruby-additional.el (ruby-mode-set-encoding): Use
  `default-buffer-file-coding-system` if the :prefer-utf-8
  property is not available.

* misc/ruby-mode.el (ruby-mode-set-encoding): Ditto.

* misc/ruby-additional.el (ruby-encoding-map): Override the
  default value.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43187 b2dd03c8-39d4-4d8f-98ff-823fe69b080e