Fix NameError for `412Error` on 412 without body

>> Aws::S3.new.head_object(if_unmodified_since: '2014-06-09 12:44:21 +0900', bucket: '...', key: '...')
NameError: wrong constant name 412Error
Merge pull request #65 from sorah/fix-412error

Fix NameError for `412Error` on 412 without body