Fix frozen string literal warning

Author: kha-wogi

lib/csv.rb

@@ -1456,7 +1456,7 @@ def generate(str=nil, **options)
       encoding = options[:encoding]
       # add a default empty String, if none was given
       if str
-        str = StringIO.new(str)
+        str = StringIO.new(+str)
         str.seek(0, IO::SEEK_END)
         str.set_encoding(encoding) if encoding
       else