[ruby/net-http] update net/http/response to be frozen string literal

https://github.com/ruby/net-http/commit/dc3b4a75ca

lib/net/http/response.rb

@@ -1,4 +1,4 @@
-# frozen_string_literal: false
+# frozen_string_literal: true
 
 # This class is the base class for \Net::HTTP response classes.
 #
@@ -273,7 +273,7 @@ class Net::HTTPResponse
 
   def error!   #:nodoc:
     message = @code
-    message += ' ' + @message.dump if @message
+    message += " #{@message.dump}" if @message
     raise error_type().new(message, self)
   end
 
@@ -648,7 +648,7 @@ class Net::HTTPResponse
     if block
       Net::ReadAdapter.new(block)
     else
-      dest || ''
+      dest || String.new
     end
   end