ActionDispatch::Request#deep_munge was introduced as a private method,
but was turned into a public one for the use of
ActionDispatch::ParamsParser.
I have extracted it into ActionDispatch::Request::Utils, so it does not
get mixed up with the Request public methods.
Use load_for_write! to ensure a refresh of the session object.
This way the new session_id and the empty data will be stored properly.
E.g. in the case of the session cookie store this means that a new
digest will be returned to the user.
