Simplify `Rack::Request#update_param`

Previously, `Rack::Request#update_param` would do an unnecessary `has_key?` check on `Rack::Request::GET`.

It used to:
1. Check if k was is self.GET and insert into self.GET if true
2. Check if k was in self.POST and insert into self.POST if true
3. If it wasn't found in either, it would be inserted into the GET params.

Why would one need to check to see if a default works? (and if it doesn't default to that default)