Asset output file name seemes to be wrong when nested true

[ippachi]

Hello.
The output file name seems to be wrong when the nested option is enabled.

Precondition

$ tree apps/web/assets/javascript

apps/web/assets/javascripts/
└── sessions
    └── form.js.es6

# apps/web/application.rb
assets do
  nested true
end

<%# apps/web/application.html.erb %>

<%= javascript 'sessions/form' %>

expected

$ tree public/assets

public/assets/
├── web
│   └── sessions
│       └── form.js
└── favicon.ico

actual

$ tree public/assets

public/assets/
├── web
│   └── sessions
│       └── form.js.es6
└── favicon.ico