Use +"" in case frozen_string_literal is enabled

Author: shugo

lib/textbringer/completion_popup.rb

@@ -174,7 +174,7 @@ def format_item(item)
     end
 
     def truncate_to_width(str, max_width)
-      result = ""
+      result = +""
       current_width = 0
       str.each_char do |char|
         char_width = Buffer.display_width(char)