csv.rb: honor encoding option

* lib/csv.rb (CSV#<<): honor explicity given encoding.  based on
  the patch by DAISUKE TANIWAKI <daisuketaniwaki AT gmail.com> at
  [ruby-core:62113].  [Bug #9766]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46391 b2dd03c8-39d4-4d8f-98ff-823fe69b080e