Remove duplicate 'uploader' when generate.

Sometimes when generating, it is really annoying.

Before this commit, when user generate uploader with
<kbd>rails generate uploader image_uploader</kbd>

User get ``app/uploaders/image_uploader_uploader.rb``

After this commit,

User get ``app/uploaders/image_uploader.rb``

Class name is fixed the same way.

This commit fix the duplicate 'uploader' problem.