Use plus operator to create unfrozen string

Author: fnordfish

lib/reline/ansi.rb

@@ -81,7 +81,7 @@ def self.set_screen_size(rows, columns)
 
   def self.cursor_pos
     begin
-      res = String.new
+      res = +''
       m = nil
       @@input.raw do |stdin|
         @@output << "\e[6n"