csv.rb: foreach enumerator

* lib/csv.rb (CSV.foreach): support enumerator.  based on a patch by
  Hanmac (Hans Mackowiak) at [ruby-core:57643].  [ruby-core:57283]
  [Feature #8929]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43135 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2013-10-04

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43136 b2dd03c8-39d4-4d8f-98ff-823fe69b080e