Suggestion: support es modules .mjs file with mime-type application/javascript

[jxck jxck]

Hi all.

Node.js decided to use `.mjs` file extension for ES Moduels.

https://github.com/nodejs/node-eps/blob/master/002-es-modules.md#32-determining-if-source-is-an-es-module

this cause developer will write es modules in .mjs file and also serve to browser as is.

so http server who determine content-type via file extension should support `.mjs` avoid to be `content-type: octet-stream`

the mime type for `.mjs` are same as `.js` as follow.

https://github.com/nodejs/node-eps/blob/master/002-es-modules.md#321-mime-of-mjs-files

so rails/actionpack/lib/action_dispatch/http/mime_types.rb better to add following line

```

Mime::Type.register "text/javascript", :js, %w( application/javascript application/x-javascript )

+ Mime::Type.register "text/javascript", :mjs, %w( application/javascript application/x-javascript )

```

any comments are welcome.

thanks

Jxck