add test of attachment thumbnail etag

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extra : convert_revision : svn%3Ae93f8b46-1217-0410-a6f0-8f06a7374b81/trunk%4013175
fix variable name confusion at AttachmentsController#thumbnail

This is suggested by Rails team.
https://github.com/rails/rails/issues/15471#issuecomment-45419147

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extra : convert_revision : svn%3Ae93f8b46-1217-0410-a6f0-8f06a7374b81/trunk%4013176