[ruby/net-http] Update lib/net/http/response.rb

@natematykiewicz's suggestion to avoid a string allocation

https://github.com/ruby/net-http/commit/925630f227

Co-authored-by: Nate Matykiewicz <natematykiewicz@gmail.com>

lib/net/http/response.rb

@@ -273,7 +273,7 @@ class Net::HTTPResponse
 
   def error!   #:nodoc:
     message = @code
-    message += " #{@message.dump}" if @message
+    message = "#{message} #{@message.dump}" if @message
     raise error_type().new(message, self)
   end