Hiki::Attachment#call should return 404 if plugin "attach.rb" is not in use
hikifarm: To avoid overwriting existing wiki, mkdir is used.
To avoid internal path name leak, rescue Errno::EEXIST.
Merge pull request #56 from ngoto/mydevel

attach.rb が無効のときでもファイルアップロードできてしまう不具合の修正
Remove needless 'then'
Remove redundant 'to_s'
Merge pull request #57 from ngoto/mydevel002

HikiFarm: Do not overwrite existing wiki