Specify external_encoding

http://c5664.rubyci.org/~chkbuild/ruby-trunk/log/20130329T090301Z.diff.html.gz

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40007 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2013-03-31

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40008 b2dd03c8-39d4-4d8f-98ff-823fe69b080e