remove useless conditional

cookies are always stored as a string in the header hash, so it will not
need to be joined.

actionpack/lib/action_dispatch/http/response.rb

@@ -381,8 +381,6 @@ def to_ary
     end
 
     def rack_response(status, header)
-      header[SET_COOKIE] = header[SET_COOKIE].join("\n") if header[SET_COOKIE].respond_to?(:join)
-
       if NO_CONTENT_CODES.include?(@status)
         header.delete CONTENT_TYPE
         header.delete 'Content-Length'