join strings rather than resizing buffer

actionpack/lib/action_view/helpers/number_helper.rb

@@ -70,11 +70,11 @@ def number_to_phone(number, options = {})
           number.slice!(0, 1) if number.starts_with?('-')
         end
 
-        str = ""
+        str = []
         str << "+#{country_code}#{delimiter}" unless country_code.blank?
         str << number
         str << " x #{extension}" unless extension.blank?
-        ERB::Util.html_escape(str)
+        ERB::Util.html_escape(str.join)
       end
 
       # Formats a +number+ into a currency string (e.g., $13.65). You can customize the format