[ruby/reline] Use plus operator to create unfrozen string

https://github.com/ruby/reline/commit/ca274b0d37

lib/reline/ansi.rb

@@ -172,7 +172,7 @@ class Reline::ANSI
 
   def self.cursor_pos
     begin
-      res = String.new
+      res = +''
       m = nil
       @@input.raw do |stdin|
         @@output << "\e[6n"