* progmodes/ruby-mode.el (ruby-encoding-map): Add a mapping from

* progmodes/ruby-mode.el (ruby-encoding-map): Add a mapping from
`japanese-cp932' to `cp932' to fix the problem where saving a
source file written in Shift_JIS twice would end up having
`coding: japanese-cp932' which Ruby could not recognize.
(ruby-mode-set-encoding): Add support for encodings mapped to nil
in `ruby-encoding-map'.
(ruby-encoding-map): Map `us-ascii' to nil by default, meaning it
doesn't need to be explicitly declared in magic comment.
(ruby-encoding-map): Add type declaration for better customize UI.